Stockfish had a rare double win in a game pair, at least one win was caused by an unusual blunder by Leela. On the other hand Leela won 3 game pairs and the score is even. The draw rate is an unprecedented low 60% so far.
Leela was up a pawn from move 16 in game 21, its eval over 1. The position was open, Stockfish had a central passer that wasn't going anywhere. Leela didn't seem to have a plan, its pawns remained in their initial squares, it moved its pieces to the first rank and then the white queen went forward alone into Stockfish territory. Stockfish's eval came down, it used the white queen as a target and took the initiative, even to make threats on the white king. Then very suddenly Stockfish's eval plunged to -4!!
Leela made a huge blunder, overlooking a simple combination (at least for a computer...). Analysis shows that after taking the bishop black can reach a winning BNN vs Q position, the trick is getting the white king to the e file and using a discovery check to win the white queen. Leela immediately saw that but one move too late. Stockfish won the rook for a bishop, the white king was under attack in the center and the white pieces pinned. Leela tried to untangle itself by exchanging pieces but lost more material, the game was adjudicated with Stockfish a rook up.
In game 22 evals were also around 1 from the start. Leela moved its king very early in the game, so it couldn't castle. It even moved its king forward to connect the rooks on the back rank. Stockfish captured a pawn and created a passer on the queen side, Leela countered on the king side and seemed to be holding, evals stayed around 1. Stockfish added a second passer on the queen side, Leela had two of its own. Then Stockfish's eval jumped over 2.5 without an obvious explanation. On move 35 Stockfish offered a pawn on the queen side.
It expected Leela to move c4+ (discovered check), exchange queens and continue to an endgame where Stockfish thought it was better. Instead Leela accepted, and suddenly the white queen attacked the king side while the black queen was too far to help. Stockfish's queen and knight were very effective, while Leela defended with two rooks. After a few moves Stockfish saw the mate and the game was adjudicated. A rare double win for Stockfish in this game pair.
In game 23 Leela had a space advantage and its eval climbed over 1.5. Stockfish remained calm and built a long pawn line across the board. Leela couldn't penetrate the line and the engines started to shuffle. After a few exchanges Leela managed to create a passer, Stockfish countered with a perpetual check draw. In the reverse game Stockfish pushed pawns on the king side, Leela countered with its queen side pawns. Evals came down to 0 and the engines exchanges pieces gradually. After a while Leela's eval became negative, still the engines shuffled and exchanged pawns and pieces, eventually reaching a B vs N ending on move 84. The game was adjudicated a draw almost 100 moves later.
Leela started game 25 with an eval around 1 while Stockfish's eval remained low. After about 30 moves evals started slowly to increase. The engines opened files and exchanged pieces, it was not clear what Leela's advantage was at this stage. On move 40 only RRB vs RRB remained, Leela's eval crossed 2.5, Stockfish's was still under 1.
The black bishop could hardly move, but it had to defend the d5 pawn and Stockfish assigned a rook to protect it. After exchanging a pair of rooks Leela's king moved forward and helped open up the king side. Stockfish's defense collapsed and Leela reduced to a winning king and pawns ending. Unusual to see Leela outplay Stockfish in an endgame.
In game 26 Leela kept its king uncastled in the center while it pushed pawns on the king side. The king side opened, Stockfish hid its king behind a black pawn and its eval fell to 0. Leela's eval turned negative and the engines started to exchange pieces. by move 39 only RRN vs RRB remained and both evals were at 0. The game reached a drawn rook ending and was adjudicated. Leela is only two points behind.
Evals were low after the start of game 27. Stockfish traded its knights for Leela's bishops early in the game, then Leela gave two pawns on the king side and evals jumped over 1. A few moves later Leela got one of the pawns back and the evals were above 2.
The threat was f6, attacking the bishop and winning the h pawn - an open h file could be deadly. Indeed f6 was played after a short delay, and in a series of exchanges Stockfish lost the bishop and the h pawn.
In a second series of exchanges the engines reduced to a RNN vs RB ending. Leela was careful to keep at least one pawn and not to exchange rooks. It took a while, Leela doesn't always pick the shortest route, but it managed to convert the win.
In the reverse game 28 there were no exchanges except a pair of pawns until move 38. The engines opened the position with a long series of exchanges, and evals dropped to 0. The game reached a RR vs RN ending with Leela two pawns up. Leela's eval turned negative but this only delayed the adjudication, draw on move 178. Two game pair wins in a row for Leela, Stockfish's lead is reduced to 1 point.
Leela's eval was over 1 after the start of game 29, Stockfish's eval remained low. The position opened after an early series of exchanges. The black queen captured the b2 pawn, this made it a target for the white rooks. Leela's eval jumped over 2 when it pushed a pawn to h6.
There was no king side attack, the black queen scrambled back and Stockfish exchanged off the minor pieces. Leela had a significant space advantage, the black pawn structure had many weaknesses. The game reached a double rook ending with evals over 4.
The kings moved forward and the rooks were all focused on the e6 pawn. Leela was able to trade the black e6 pawn with its f pawn and to push a white pawn to e6. The black pieces could hardly move, Stockfish saw the mate coming and the game was adjudicated.In game 30 the position remained closed. Stockfish started with an eval around 1 but it remained there as the engines mainly shuffled. Leela's evals dropped below 0 after a series of exchanges on the queen side. The engines continued to shuffle for a very long time, moving a pawn or exchanging pieces occasionally to reset the 50-move counter. Only after the position opened, both engines queened and the checks started, did both engines lower their evals enough for the draw rule. The game lasted for 233 moves. Leela evens the score after 3 straight game pair wins.
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