Season 18 superfinal games 51-60

After 60 games Stockfish leads 14-9 with 37 draws. Stockfish wons 3 game pairs, increasing its lead significantly. Leela should have won a game pair but it blundered into a very deep winning line that only Stockfish saw.

Stockfish moved its king without castling early in game 51. Leela was a pawn down but had an eval advantage, the black king side rook was trapped in the corner. The engines exchanged minor pieces and Leela regained the pawn. Stockfish released its trapped rook, the game reached a QRB vs QRB position on move 30, Leela had a queen side passer. After exchanging queens Stockfish used its pieces to prevent the passer from advancing After some shuffling the engines exchanged rooks and reached an opposite color bishop ending. Leela was two pawns up but it was still a draw, the game was adjudicated on move 107.

Leela did not castle its king in game 52, the black pieces were mostly on the back rank and the engines played on the queen side. The b file was half open and Stockfish placed a rook there, its eval started to increase when it added a second rook and threatened the black b6 pawn. Leela couldn't defend the pawn, its eval was still relatively low.

After Stockfish captured the b6 pawn the engines exchanged most pieces. Leela soon realized it was in trouble, in a RB vs RB position its rook was still trapped on the king side. By the time the rook broke out it was still on the wrong side of the board. Stockfish gave its bishop and pushed a pawn to the 7th rank, Leela lost its rook to stop it. 

The rook was much stronger in this position, Leela tried to protect its a pawn but Stockfish just traded the rook for the bishop. The white a pawn became a passer and raced forward, promoting first for the win. Stockfish wins a game pair, its lead is now +3.

Games 53-54 started with a 23-ply book, a popular line in the King's Indian, Orthodox variation. Leela pushed a pawn to a6 in game 53, Stockfish pushed pawns on the king side. The engines exchanged all bishops, then Leela's queen moved deep into black territory. Leela captured a pawn but it could not retreat. Stockfish gave a series of queen "checks", Leela's eval came down and the game was adjudicated. In game 54 the setup was similar. Stockfish's eval went over 1 for a while after it opened the queen side and went a pawn up. Leela regained the pawn, evals came down and the engines exchanged bishops and a pair of rooks. The game ended in a 3-fold repetition.

In game 55 the engines castled in opposite directions. Starting from move 10 there was a series of moves following long agreements in the engines' PVs. Most pieces were exchanged and the game reached a RB vs RB position with white a pawn up on move 22. Evals slowly increased, Stockfish moved its king to protect the queen side pawns, its pieces stayed on the king side where Leela had a pawn majority. The engines exchanged rooks, leaving a same color bishop ending. Stockfish's eval was over 4 for a few moves, Leela couldn't find the win and evals came down. The rest of the game was a long shuffle with a few pawns moves and exchanges. The game was adjudicated on move 191.

The engines castled in opposite directions again in game 56. Again there was a series of exchanges with long PV agreements. On move 22 only RRN vs RRB remained, and Stockfish went a pawn up.

Stockfish exchanged a pair of rooks and its eval jumped over 2. The engines traded pawns and Stockfish's eval continued to increase quickly, Leela's eval also started to increase. Stockfish managed to capture all the black pawns and it had two pawns left, one on each side.

The game continued for another 30 moves. Leela delayed the advancing white pawns as much as it could, Stockfish pushed one forward and then the other. Eventually Stockfish found a way to capture the bishop and the game was adjudicated. Stockfish wins the game pair and the lead increases to +4.

Stockfish castled long in game 57, Leela did not castle at all. There were no pawn exchanges after the start, Stockfish's eval came down to 0 and stayed there for the rest of the game. The engines opened a file on the king side and after some shuffling exchanged a pair of rooks. There was a long shuffle starting from move 32, with one pawn move. A few exchanges led to a QRN vs QRB position, then shuffling resumed. Leela lowered its eval for the draw rule on move 126.

The kings' setup was different in game 58, Stockfish as white castled long and Leela castled short. Leela pushed pawns on the queen side while Stockfish gave a pawn and opened a file on the king side, both evals came down. Leela started to plan how to get its king out of the exposed king side, meanwhile it moved its queen to protect a pawn and Stockfish thought that Leela chose the wrong square. It gave a second pawn and opened the center, its eval jumping to around 2.

The black king was out in the open and Stockfish chased it all the way to the b5 square. Leela predicted all these moves in advance yet its eval stayed low. Stockfish had to be careful not to get mated on the back rank, the attack ended with a series of exchanges that left a Q vs RR position.

It took Leela a while to see it was losing. Stockfish pushed the f pawn forward and captured the remaining black pawns. Leela's king was pushed back and Stockfish pushed a second pawn forward before the game was adjudicated. Stockfish wins another game pair, the lead is now +5.

In game 59 Leela appeared to sacrifice its queen for a knight almost immediately at the start. In fact this was the beginning of a combination that included taking the black queen with a pawn. On move 11 Leela was a pawn up with evals over 1.

Leela sent its king to the queen side and developed its queen side pieces. Stockfish gave a pawn to break up the white queen side pawns, its king moved to the king side where there was only one pawn left. The game reached a RRN vs RRN position, Stockfish got one pawn back while evals continued to increase. The engines traded a pair of rooks, all the black pawns were isolated and Stockfish tried to keep them alive.

The white rook moved forward and Stockfish couldn't stop it from taking the queen side pawns. Leela was 3 pawns up with connected passers and the game was adjudicated.

Game 60 started much more quietly. Leela castled its king, Stockfish decided not to since both sides were too exposed. Leela opened a file on the queen side, exchanged a pair of rooks and went a pawn up. Evals remained low, the engines traded R for BN and the black queen moved forward. On move 32 Leela moved its queen, immediately Stockfish's eval jumped to 24 (!!). Leela's eval predicted the next 7 moves correctly, but its eval remained low. Stockfish sacrificed a bishop and exposed the black king.

Stockfish presented 100% tablebase win scores, Leela remained confident it could hold despite the white queen being so close to its king. Stockfish exchanged bishops and moved the passer to e7, Leela had to block with a rook. Stockfish was careful not to allow a perpetual check threat, the knight came forward to protect the passer and threaten mate. Leela soon ran out of checks, its eval was over 2 and it again dropped, really having trouble in understanding the position.

Stockfish captured a pawn and exchanged queens, Leela tried to race to the line but Stockfish threatened mate in the corner, captured the rook and promoted first with check. There were two white wins in this opening, though Leela should have held the reverse. Stockfish's lead remains +5.

Season 18 superfinal games 41-50

After 50 games Stockfish leads 10-8 with 32 draws. Leela got within 1 point twice, and Stockfish won the reverse game in both game pairs. At the halfway point of the match Stockfish keeps a 2 point lead, still quite close and anything can happen. 

Games 41-42 started with a Scotch game modern defense variant, the black queen came forward and the black king moved and could not castle. In game 41 there were many early exchanges and only QRB vs QRB remained on move 18.

Evals slowly increased, Leela was a pawn down but had a better pawn structure, especially on the king side where Stockfish had doubled pawns. The engines exchanged queens and Leela managed to capture all the black pawns on the king side. 

Leela was careful and patient with its king side pawns, after 15 moves Stockfish decided to give its bishop and remove them. Leela had enough pawns left to win a RB vs R ending.

The pieces stayed on the board a bit longer in game 42. After exchanging queens Stockfish had a bishop pair, Leela went a pawn up but Stockfish's eval started to increase. Stockfish traded a bishop for a knight and only RRB vs RRN remained on move 27, Stockfish's eval was over 2.

Stockfish captured a pawn on the king side and created a 3 to 1 pawn majority. The engines exchanged pawns and Leela created 2 passers on the queen side. It pushed a pawn to the 2nd rank and its low eval showed it thought it was relatively safe. Stockfish's eval continued to increase, it controlled the 7th rank with a rook and it was sure it was going to win.

Leela couldn't hold on to the d pawn after the white king moved towards it. The other black passer was too far to be a serious threat, Stockfish doubled rooks on the 7th rank and Leela realized it was going to lose.

Leela avoided an immediate mate but Stockfish captured the knight a few moves later. The game was adjudicated in a RB vs R position with one white pawn to ensure the win. There were two white wins in this opening, lead is still +2.

Game 43 reached a R vs NN imbalance early. Leela was two pawns up, the engines exchanged queens and a pair of rooks and the game reached a RB vs BBN position on move 32. Stockfish's eval came down to 0, the most Leela could do was to give a pawn a get a passer to the 7th rank. Leela continued to play, the game was adjudicated in a R vs BN position on move 78. In game 44 Leela pushed pawns in the center and planted a knight on the 3rd rank. Stockfish exchanged a bishop for the knight and cleared the black central pawns. After a series of exchanges the game reached a RN vs RN position on move 35. Both engines had a pawn near promotion, after a while they traded knights and captured the passers leading to a drawn rook ending. 

In game 45 Leela pushed pawns in the center and Stockfish pushed pawns on the queen side. Evals mostly stayed below 1, Leela waited until move 20 to castle though its king had low pawn cover. Leela surprised Stockfish on move 23 with a pawn move to f5. The white king side completely opened, and then Leela gave a pawn and created an advanced passer in the center. Stockfish's eval jumped over 2, a series of exchanges led to a RBN vs RBN position.

The black knight was almost trapped, Stockfish lost two pawns to save it. While Stockfish concentrated on getting rid of the passer Leela captured another pawn, it was two pawns up in a RN vs RB position on move 49.

Leela drove the black king back and captured another pawn, the game was adjudicated soon after that.

In game 46 Stockfish pushed pawns on the king side and castled short like the previous game. There were several early minor piece exchanges and queens were also off the board. Stockfish's eval started to increase after Leela grabbed a pawn on the queen side and allowed Stockfish to create a pawn majority on the king side. The white king moved forward supporting its pawns and Stockfish placed a bishop and pawn pair deep in black territory. After exchanging a pair of rooks the game reached a RBN vs RBB position and Stockfish went two pawns up.

The black bishop pair was strong, Stockfish's passer could not advance and it was not clear how Stockfish can win, though its eval continued to increase. On move 50 Stockfish traded its knight for a black bishop, then traded rooks and gave its passer to take the black h pawn.

The result was an opposite color bishop ending. Usually a 2 pawn advantage is not sufficient in such an position but this was an exception. The connected pawns needed the attention of both black king and bishop, and by the time Leela captured one pawn Stockfish pushed the c pawn to the 6th rank. Leela could not stop both white passers and Stockfish promoted one to a queen, winning the game. There were two white wins in this opening, lead is still +2.

Leela opened the king side and castled long in game 47. Stockfish exchanged queens and opened the queen side, its eval dropping to 0 though Leela was a pawn up. Only rooks and bishops remained on move 29, both kings were exposed to checks. Leela avoided a repetition draw but the draw rule adjudicated the game. Game 48 started with a similar setup with Stockfish castling long. The white queen came forward and Stockfish tried a king side attack. Leela defended well, it tried to exchange queens but Stockfish refused. After several attempts there was a series of exchanges instead and the game reached a QN vs QB position on move 35. Evals were low, the game continued for 35 moves and was adjudicated when the pawns stopped moving.

Black had an advantage from the opening in games 49-50. In game 49 all the bishops were exchanged by move 16, with all pawns still on the board. Leela didn't castle its king, instead the king walked to the king side. Stockfish's eval came down and it mostly played waiting moves. The engines opened a file on the queen side and exchanged a pair of rooks, after that the game was adjudicated by the draw rule. In game 50 Stockfish castled early and the engines exchanged pawns with all pieces on the board. Leela didn't castle as black, starting from move 21 the engines exchanged pieces and on move 29 only RB vs RB were left with Stockfish a pawn up. The engines mostly shuffled with some pawn moves or captures, Leela slowly lowered its eval. The game was finally adjudicated on move 103, close to a 50 move draw. 

Season 18 superfinal games 31-40

After 40 games Stockfish leads 8-6 with 26 draws. Only one decisive game, Leela wins a game pair and Stockfish's lead is +2. The match is not so one-sided as it was in the first 10 games, Leela is still fighting.

A long series of early exchanges left only RRB vs RRB on move 21 in game 31. The engines stabilized the pawns and then exchanged a pair of rooks. A second series of exchanges reduced to an opposite color bishop ending, Leela had connected passers and Stockfish had a passer that reached the 2nd rank. Leela thought it had an advantage, it took a long time for it to lower its eval for the draw rule on move 96. The pieces stayed on the board longer in game 32, the engines cleared the queen side pawns and exchanged queens and minor pieces. Evals came down to 0 and the game was adjudicated when the pawns stopped moving.

The center opened early in game 33 and the engines exchanged most pieces. Evals climbed over 1.5, on move 25 only BB vs R remained. Stockfish was a pawn up but it had two weak pawns on the queen side and only one piece to defend them. 

The bishop pair was very strong, Stockfish played passively and tried to protect its pawns. Leela's king came forward targeting the black a pawn. There was no room for the rook and king as defenders and Stockfish lost the a pawn on move 46.

The black rook became active again, but it couldn't attack on its own and the white pawns were protected. Leela slowly pushed the a pawn forward, with its king and bishops supporting. The game was adjudicated when the white king found a way to move behind the black pawns.

In game 34 the black queen came forward early, Leela traded it for two bishops and a knight. Stockfish captured two pawns and its eval went over 2 and kept increasing. Both engines attacked and the position became chaotic, Stockfish threatened mate on the king side while Leela pushed two pawns on the queen side facing the white king. After material sacrifices from both engines the game settled down in a Q vs RN endgame on move 39. 

Despite the high eval Stockfish could not find a way to win here. Leela abandoned the a pawn and placed its rook on the 6th rank. Together the rook and knight controlled the d6 square, stopping the white pawn. Stockfish got nowhere with its queen alone and it gave up trying as the 50 move draw approached. Leela wins a game pair and Stockfish's lead is +2.

Leela pushed pawns on the queen side in game 35, despite castling long. A file opened in the center and the engines exchanged queens and a pair of bishops. Evals were low and the engines started to shuffle. There were a few more piece exchanges and pawn moves, the game was adjudicated when Leela lowered its eval for the draw rule. In game 36 Stockfish castled short and Leela pushed a pawn to h3. Stockfish went a pawn up, its eval peaked at 1.5 and then came back down. The engines cleared most of the pawns on the queen side, then shuffled for a while. Evals were close to 0 and the game was adjudicated on move 35, in a position with many pieces and pawns on the board. Too soon?

Leela pushed a pawn to h6 in game 37, and Stockfish castled short nevertheless. The engines exchanged queens and all bishops, that made the black king safe. After exchanging a pair of pawns the engines started to shuffle on move 35. Evals were around 1 but neither engine moved a pawn and the game ended in a 50-move draw. Game 38 started similarly with a white pawn on h6 and Leela castling short. Stockfish attacked the king side, capturing two pawns and making its h pawn a passer. Leela hid its king behind the passer and exchanged queens. Leela avoided opening a file on the king side and exchanged bishops, its king was trapped in the corner but somehow safe. Evals came down and the game was adjudicated in a RRN vs RRN position, Leela's PV showed a drawn double rook ending.

In game 39 Leela pushed pawns on the king side and its king stayed uncastled in the center. Stockfish opened a file on the king side and gave a rook for a bishop. The white king was exposed and Leela quickly exchanged queens, Stockfish's eval came down to 0. The game reached a RRN vs RBN position, and the engines gradually exchanged pawns until none were left. The material advantage was not enough for a win, Leela was slow to lower its eval and the game was adjudicated on move 90. In game 40 again Stockfish pushed pawns on the king side, it did castle its king but without pawns the king was still exposed. Leela gave two pawns and opened a file in the center, its eval turned negative after it doubled rooks in the file. Leela went on the attack with many threats and pins, however Stockfish's eval was at 0. After most pieces were exchanged the game reached a RB vs Q position with white two pawns up. Leela could only give checks, after 25 moves it lowered its eval for the draw rule.

Season 18 superfinal games 21-30

After 30 games Stockfish leads 8-5 with 17 draws. Both engines won a game pair, first game pair win for Leela. Stockfish continues to lead by 3 points.

Stockfish gave a pawn early in game 21, then the engines reduced to a RB vs RB position on move 21. Leela created a queen side passer and Stockfish blocked it with its pieces. The white king could not move forward and the engines shuffled for a long time. Stockfish ended the game with a perpetual check. In game 22 all the pieces were on the board on move 25 except for one white pawn and a pair of bishops. The white queen went forward and captured two pawns, Leela opened the king side with a rook sacrifice and attacked the white king. A long series of exchanges led to a N vs BN position with white two pawns up and low evals. The game continued for 60 more moves with pawn moves and exchanges before it was adjudicated.

Games 23-24 started with a 20-ply book in the Queen's indian, Petrosian variation, with black a pawn up.

In game 23 evals increased slowly after the start, and most minor pieces were exchanged. Leela opened a file on the queen side and doubled its rooks there. Evals peaked at around 1.5, then the pawns stabilized and the engines started to shuffle. After 35 moves a pair of rooks was exchanged and then queens as well. The engines continued to shuffle in a RN vs RB position, evals slowly came down. The rooks were eventually exchanged as well, the game was adjudicated on move 175.

There were less exchanges after the start of game 24, Stockfish's eval stayed around 1. The black pieces were pushed back and Stockfish had more space, but it didn't seem to have a clear plan. After move 30 Leela played a few moves that were inaccurate according to Stockfish, on move 35 Stockfish's eval jumped over 3.

The white queen had a diagonal line of sight to the black king, Stockfish used the check to move the queen forward on the queen side, and a rook followed in the center. The attack on the black king included a temporary rook sacrifice, the attack was over in a QN vs RRB position on move 48. The material was close to equal yet Stockfish's eval was close to 10. Leela's eval was much lower, still hopeful.

The queen danced across the board, giving checks and preventing any counter Leela might try. After 20 moves Stockfish captured a rook and the game was adjudicated. Stockfish wins the game pair, its lead increases to +4.

Stockfish was a pawn up early in game 25, evals were low around 0.5. Stockfish moved its king side rook and decided not to castle its king at all. Evals increased slowly as Leela increased the pressure on the king side. It pushed a pawn to h6 and threatened mate, Stockfish saved its king but its king side rook was trapped in the corner.

Leela traded its bishop for the rook, and the black queen got trapped in the corner. Leela's queen came forward and captured two pawns before Stockfish could untangle itself.

The game continued for a few more moves, but the material advantage gave Leela the win.

As before Leela was a pawn up in game 26. This time Leela chose to castle long despite the partial pawn cover on the queen side, and its king side rook was free to move. Stockfish's eval was around 1 as it slowly moved pieces to the queen side. Leela doubled rooks on the g file and created pressure on the g2 pawn and the white king. Stockfish had to defend carefully, its position seemed very hard to hold, yet its eval went over 2.

Leela saw its attack wasn't going anywhere but it was difficult to retreat, it tried to exchange queens but Stockfish refused. Stockfish opened another file on the queen side and its two rooks trapped the black king, preparing for the final attack.

Stockfish traded a rook for the defending knight, the white queen left the g2 pawn and went on the atttack. Leela was forced to give its queen for a rook to avoid immediate mate, but it couldn't save the game. The white knight joined the attack and Stockfish won more material to win. Two white wins in this game pair, no change in the lead.

Games 27-28 started with black a pawn up. Stockfish's eval started at (negative) 1 in game 27, then came down after the engines exchanged a few minor pieces. After exchanging queens Leela had a bishop pair to compensate for the pawn. Evals were close to 0 after move 20, the engines continued to exchange pieces and Leela regained the pawn. In a king and pawns ending both engines promoted a pawn to a queen, the game ended in perpetual check. In game 28 Stockfish's eval again reached (negative) 1, then came down when the engines started a series of exchanges. By move 33 the game reached a knight ending, with adjudication 10 moves later.

Leela pushed pawns on the queen side in game 29. The black pieces were pushed back, Leela opened a file and one of the pawns reached a6. Evals slowly increased as the engines exchanged pieces and pawns, Stockfish couldn't get rid of the white a pawn. On move 31 the game reached a RB vs RN position.

Stockfish could have taken the a pawn but chose not to do it since that would bring the white rook to the 7th rank. Instead Stockfish exchanged the minor pieces and reduced to a rook ending, its eval jumping as it realized it was losing. The kings moved forward and the pawns stabilized, Stockfish found itself in near zugzwang. Leela pushed a pawn to g6, creating a second entry point to the 7th rank for its rook. Leela's eval went over 2.

The white king moved to the queen side and threatened to penetrate. Stockfish had to move its rook to stop the king, allowing Leela to capture the e pawn. Leela took over the 7th rank, another black pawn was captured and the game was adjudicated.

In game 30 Stockfish went a pawn up tried to attack on the king side but was not strong enough. The engines exchanged pieces, Leela regained the pawn and evals came down to 0. The exchanges continued until the game reached a drawn rook ending. Leela wins a game pair for the first time in the match, the lead is back to +3.

Season 18 superfinal games 11-20

After 20 games Stockfish leads 6-3 with 11 draws. There were two openings with two white wins each, Stockfish's lead is unchanged. Leela's performance is still a hot topic in the chat. My understanding is that a combination of Leela's software update and the way it uses the CPU on the GPU machine is making it 20% slower in the superfinal. It is not clear how much weaker this makes Leela, if at all. I think it is mostly a small sample issue. In any case I prefer to focus on the games.

The engines exchanged most minor pieces early in game 11. Stockfish pushed a pawn to f3 while Leela took over the center, the position opened and Stockfish went a pawn up. The game reached a QR vs QR position with low evals, the engines mainly shuffled or gave checks until the game was adjudicated. In game 12 again most minors were exchanged early, though the position was different. The game reached a QRR vs QRR position on move 28 with most pawns still on the board. The engines shuffled and evals came down, then the position opened and reached a queen ending. Stockfish ended the game with perpetual check.

In game 13 Stockfish chose to have a BNN vs Q imbalance early. Stockfish's eval was negative, it thought it had an advantage. The black king walked to the corner, Leela could take a rook for a bishop for many moves but it wouldn't give up its bishop. By the time the rook moved to safety evals became positive. All the black pawns were on the board and the black heavy pieces were not effective behind the pawn line. The engines opened the pawn wall on the king side, evals increased over 2 as Leela placed two minor pieces that blocked the pawns in the center and further limited the black piece movement.

Stockfish gave a rook for a knight and opened the position. Both engines had advanced passers on the king side. Stockfish's queen was strong enough to keep several white pieces busy blocking the black passer. Eventually Stockfish lost the passer while reducing to a RBB vs RB position, evals were very high.

Leela had control of the dark squares, its DS bishop made sure the black pawns could not advance. The white king came forward, probably planning to attack the c6 pawn. The game continued for 20 moves before it was adjudicated.

Leela played more peacefully in game 14 and evals were close to 0. Leela had a bishop pair advantage and it went a pawn up on move 20, Stockfish controlled the center and its two knights were deep in black territory, one on the 7th rank. On move 27 Stockfish's eval suddenly jumped over 3.5, it seemed to gift a knight and sacrifice a rook but then it captured the queen and a bishop after a discovered check. The game reached a QR vs RRB position on move 33.

Stockfish was sure it was winning, Leela was not convinced. Stockfish protected the back rank to avoid mate, and it slowly pushed the b pawn forward. Leela countered with the d pawn, Stockfish captured the pawn and the engines exchanged a pair of rooks. 

Stockfish had to juggle between giving checks and avoiding a back rank mate. It took about 20 moves to force the rook to leave the 8th rank and then Stockfish was able to promote its pawn and the game was adjudicated. There were two white wins in this opening, but the games were very different and the opening doesn't seem to be biased.

Games 15-16 started with the Budapest defense, white a pawn up. In game 15 the engines opened a file on the king side, exchanged a pair of rooks and the white queen came forward. A series of early exchanges led to a BNN vs BNN position with white a pawn up. Evals were over 1 for a while, Stockfish reduced to a same color bishop ending and moved its remaining pawns out of the bishop's reach. Leela's king could not move forward to attack the pawns and the engines shuffled for a long time. When Leela used its pawns to open a path for its king there were not enough pawns to win. In game 16 Leela avoided opening the king side and it regained the pawn early. Stockfish moved its queen forward, it captured 3 pawns on the queen side and then was traded for two black rooks. Evals came down, Leela opened the king side and threatened the white king. Both engines thought there was nothing to play for and the game was adjudicated on move 40, still with many pieces on the board. Both engines managed to hold in a difficult opening.

Games 17-18 started with a 31-ply book sequence, a popular line in the Sicilian Najdorf variation. White had a pawn on f6 and the g file was open, and the black king was stuck in the center. In game 17 Leela moved a rook to g7, daring Stockfish to exchange it for a bishop so its pawn would advance to g7. Stockfish refused and tried to attack on the queen side, Leela traded BN for R and stopped the attack. Leela created an advanced passer in the center, the black king's situation looked scary.

Stockfish had no choice but to capture the passer, opening the d file. The black king tried to find cover on the queen side, protected only by pieces. Leela used a queen exchange to get its f pawn to the 7th rank in a RR vs RBN position.

Stockfish captured the e pawn at the cost of losing its remaining pawns. The rook pair was very strong on an open board, eventually Leela captured the knight and the game was adjudicated.

In game 18 Stockfish's eval shot up and was already over 7 after playing 4 moves. It sacrificed a knight in the center and pushed its pawn to f7, forcing the black king to move. Leela captured the pawn, it was up a piece but its king was almost defenseless in the center.

Stockfish sacrificed its bishop and cleared the way for its rook to join the attack. The white queen and rook attacked together, chasing the black king and regaining material. When the attack was over only N vs BN remained, Stockfish 3 pawns up.

The white queen side pawns were unstoppable. Stockfish pushed them forward, Leela gave the bishop to stop a promotion and the knight ending was a win for Stockfish. There were two white wins in this opening, the evals suggest it was too biased.

The first capture in game 19 was on move 20, then a series of exchanges opened the position. The engines shuffled for a while and evals were low, Stockfish's eval jumped over 1 when Leela didn't retake a pawn and went on the attack instead. The engines exchanged queens and Stockfish captured a second pawn, its king was threatened by the white rooks and bishops together with a pawn on h6. After a series of exchanges the game reached a RB vs RB position, Leela went a pawn up with a passer. Stockfish's pieces prevented the passer advancing and the engines shuffled, evals came down and eventually Leela just gave the passer up. After a second long shuffle the engines reduced to a drawn 7-man position and the game was adjudicated. In game 20 Stockfish castled long and concentrated on the king side. After a few exchanges evals came down, Stockfish opened a file on the king side and Leela exchanged a pair of rooks. Stockfish doubled rook and queen and threatened the black king, Leela traded RB for Q and reached a QN vs RBN position. The queen side was locked, the engines exchanged knights and the white queen could not win the game on its own. The game was adjudicated a few moves later.

Season 18 superfinal games 1-10

After 10 games Stockfish leads 4-1 with 5 draws. This result is quite unexpected, considering that Leela beat Stockfish last season, not too long ago. The chat is buzzing, with fans of the two engines arguing and trying to figure out what is going on. Various explanations have been suggested, including the engine software updates, Leela's configuration and the upgraded hardware. It could be a random result of the small sample size, likewise for the result of the previous superfinal, but who wants to believe in statistics? Another contributing factor is the openings. As Jeroen explained, the openings for this superfinal are more biased as he preferred more decisive games, at the cost of having more openings with wins for both engines. Perhaps Stockfish is better in such openings? More decisive games means more work for me, but I'm not complaining too hard.

Games 1-2 started with a Sicilian 2-ply opening. Game 1 continued in the Kan Polugaevsky variation. There were very few exchanges and evals stayed low. Stockfish started shuffling before move 20, after a few exchanges on move 29 both engines continued to shuffle and the game was adjudicated. In game 2 the engines played a B50 Sicilian with d6 and d3 played. There were many early exchanges and only QRB vs QRN remained on move 24. Evals were close to 0 and the game was adjudicated before move 40. 

Leela had an eval advantage from the start of game 3, Stockfish seemed to be holding in the first 20 moves. The center was blocked, Leela pushed pawns across the board and kept its king uncastled. The first exchange was on move 20, Stockfish prepared a king side attack and Leela concentrated on the queen side. Stockfish's eval went over 1, on move 27 it moved its queen away from defending the b7 pawn, but it didn't expect Leela to take it. 

Leela captured the pawn and Stockfish's eval immediately jumped as it saw the blunder (so why did it make the move in the first place??). Leela traded Q for RR, then a quick series of exchanges led to a RB vs BN ending, white a pawn up.

Stockfish had a passer that had to be watched but the material was enough for Leela to win.

In game 4 Stockfish pushed pawns on the king side and castled long. It prepared a king side attack, doubling rooks on the g file, and its eval went over 2. Leela refused a bishop sacrifice but it couldn't prevent the king side being opened. Leela moved its pieces to surround the king, that was left without pawns.

Stockfish shifted its rooks to the open e file, and created a passer on the queen side to increase the pressure. It gave a rook for a knight and prepared for the final attack. 

The white rook and queen moved forward, Leela defended its king and exchanged pieces until only QN vs QB were left. Stockfish was only a pawn up, but after Leela ran out of checks Stockfish forced a queen exchange and the b passer was unstoppable. There were two white wins in this opening, score is 1-1.

Stockfish moved its king early in game 5, out of known theory. Evals increased over 1, the engines opened a file on the queen side, Stockfish stabilized its pawns and started to shuffle. Leela couldn't find an entry point, it offered a knight but Stockfish wouldn't open the king side. Evals slowly came down, after exchanging a few pieces Leela started to shuffle as well, the game ended with a 50-move draw.

Game 6 continued along a more popular line, Stockfish's eval was around 1.5. Leela gave a pawn early to activate its pieces and create a king side threat. Stockfish concentrated on the queen side, it opened a file and placed a knight on c5. Leela created a queen side passer and gave a rook for a knight and pawn to remove the white knight. Stockfish's eval was over 2.

Stockfish moved its heavy pieces to the queen side, it blocked the passer with a rook while taking over the open file. Leela played passively and evals continued to increase .On move 41 Stockfish exposed the black king with a rook sacrifice. 

The white queen came forward, it chased the black king while capturing pawns. Stockfish was 3 pawns up when it exchanged queens with only RB vs RNN remaining. Leela couldn't stop the white king side pawns marching and the game was adjudicated. Stockfish wins first game pair and goes into the lead. 

There was a series of early exchanges in game 7, Leela was up a rook for a knight while Stockfish was two pawns up with connected passers on the queen side. Stockfish's eval came down to 0, Leela's king blocked the passers while its pieces couldn't find a way through the black defense. The position opened with more exchanges, Stockfish gave a piece and its queen side passers and Leela was left without pawns in a RB vs N position. Stockfish had two connected pawns, close enough to promotion so they were worth a rook. The game ended in a tablebase draw. 

In game 8 there were more pieces on the board. Stockfish thought Leela's 15th move was a mistake and its eval jumped over 1.5. It immediately gave two minors for 3 pawns and its eval jumped higher while Leela was completely calm. After exchanging queens the center opened, Stockfish captured two more pawns and created a queen side passer. The white rooks controlled the center and the black king was exposed, yet Leela's eval remained close to 0.

Leela realized something was wrong only on move 27. It gave back some of the material but couldn't save its king. Stockfish attacked with its rooks and managed to capture a black rook.

Leela's last hope was its passer, Stockfish traded a rook for a bishop to get rid of it. The rook ending with 3 pawns up was as easy win. Such engine blindness in a superfinal is a surprise to me, I don't think there was such an extreme case in the last several seasons. Stockfish wins the game pair and increases the lead to +2.

Games 9-10 started with a King's Gambit accepted, with black up a knight for a pawn. In game 9 there were a few early minor piece exchanges. Stockfish was late in developing its king side and its king was uncastled in the center. Leela moved its heavy pieces to the half open f file, Stockfish prepared an escape route for its king to the queen side. Stockfish's (negative) eval increased steadily and was over 2.5 on move 26.

Leela moved its rooks forward, Stockfish captured pawns while Leela shifted its pieces to threaten the king on the queen side. In a series of exchanges Leela regained the piece and reduced to a queen ending with black a pawn up. Despite the small material advantage Stockfish's eval was close to 20 by then, Leela was more hopeful.

The ending was far from trivial and the game continued for more than 25 moves. After a few pawn exchanges Stockfish pushed its queen side pawns forward. The black king had more cover and the white king was exposed to checks, Stockfish used this to get a pawn to the 2nd rank supported by its king. When the second pawn moved forward the game was adjudicated.

In game 10 both engines castled long early and evals came down. The black queen moved forward, Leela traded it for a rook and bishop to get a Q vs RBN imbalance on move 17. The white queen was very active, Leela's pieces defended each other and did not try to attack. Stockfish opened the queen side and threatened the black king, the engines mostly shuffled and the game was adjudicated when the pawns stopped moving. Stockfish wins another game pair and the lead is +3.

Season 18 premier division statistics

A summary statistics table of previous stages and seasons.  

Draw rate, wins

Final draw rate was 79.2%. 

Game termination

The three most common game termination causes were:

51.8% - TCEC draw rule
19.6% - TCEC win rule
17.3% - SyzygyTB

There were no crashes in the stage.

Moves per game

Median= 57.5
Average= 68.7

There were 23 games longer than 100 moves, the longest was 304 moves (rofChade - Leela, game 141, draw). 

Time per game (hours)

Median= 2:57
Average= 2:54

Openings

There were 8-move book openings in this stage chosen by Cato. The first letter of the ECO codes was distributed as follows:

The engines had almost no freedom to choose the opening variant, all except two of the game pairs repeated the same ECO code and the same opening variant twice. In all of the game pairs the ECO first letter was repeated twice.

Reverse pairs, wins

Reverse pairs, same moves 

Pairs of reverse games diverged very quickly, 32.1% diverged immediately out of book, 67.9% of the pairs diverged at most after 1 move. The longest repeated sequence of moves was 16 plys (Stoofvlees - Leela games 75 and 103, Alekhine's defence, modern, Larsen variation, two draws).