Season 24 superfinal games 41-50

After 50 games Stockfish leads 10-9 with 31 draws. Stockfish is back in the lead after two game pair wins. There were two game pairs with two draws and one with two white wins. At the half way point the match can still be won by both engines, a tiebreak is also a possibility.

Stockfish's win in game 46 came after a very long queen ending, with Leela's eval below 1 for 30 moves, even when Stockfish already saw a tablebase win in its PV. I haven't seen such a discrepancy of evals for a very long time, and it is even more surprising in a superfinal. 

Games 41-42 started with an 18-ply long rare sideline in the KID Fianchetto variation. In game 41 there were many early exchanges played in PV agreement after the start. The game reached a QRR vs QRR position with white a pawn up on move 25. Leela had a pawn majority on the queen side, the king side pawns stabilized and the engines exchanged a pair of rooks. Evals came slowly down, Leela created a passer and pushed it forward. Stockfish countered by exposing the white king and reducing to a drawn queen ending. In game 42 there were no exchanges after the start except one pawn exchange on the king side. The black queen moved forward on the king side, starting from move 19 the engines played out a long PV agreement, exchanging pawns and minor pieces in the center. Evals came down with most pieces still on the board. The engines gradually exchanged pieces and the game was adjudicated in a RN vs RN position.

Games 43-44 started with a rare sideline of the French Advance variation, rarely seen in recent human games. In game 43 the engines exchanged a pair of bishops after the start, Leela pushed the h pawn and moved its king without castling. The engines traded minor pieces on the king side, Stockfish kept its king in the center uncastled as well. Leela went a pawn up and exchanged queens, evals started to come down. The engines exchanged pieces until reaching a RN vs RN position and the game was adjudicated. 

In game 44 Stockfish pushed pawns on the king side, again the engines exchanged a pair of bishops but there were no other exchanges after the start. Leela pushed passers on the queen side and then castled long, a pawn exchange opened the a file. The engines formed a diagonal pawn wall blocking the center and queen side except the open a file, Stockfish castled short. Pawn exchanges opened the king side, each engine created a passer. All the rooks were exchanged through the open files, evals remained well below 1 but Stockfish's eval started to increase after move 39. Leela's eval stayed under 1 for 4 moves while Stockfish's eval jumped over 2, on move 48 Stockish captured the black passer.

The queen side was empty of white pieces, perhaps Leela thought it would have an effective counter but now its eval jumped as well. Leela sacrificed a knight for a pawn and the b pawn became a passer on the 2nd rank. Stockfish rushed back with its queen to block, then slowly sent its other pieces back to assist. The white king became exposed and Leela delayed with checks. Eventually Stockfish captured the passer on move 72. 

Now Stockfish was a knight and pawn up with no compensation for Leela. Stockfish gave the passer to exchange queens, the game reached a BN vs N position. It took another 15 moves before the game ended in a tablebase win. Stockfish wins the game pair, the score is 8-8.

Games 45-46 started with a rare sideline of the closed Sicilian defense. In game 45 the engines exchanged pawns and minor pieces after the start, the white central pawns were cleared and the black pawn structure was weaker than the white. Both engines castled short, on move 19 Stockfish moved its queen forward on the queen side and attacked the a pawn. Leela abandoned its a pawn and pushed the h pawn forward. Stockish's eval came down as it took the a pawn, it felt confident its queen can retreat in time to help on the king side. Leela pushed the h pawn forward, at first its eval increased but then came back down as the engines followed a long PV agreement. Leela opened the king side and exposed the black king, the black queen covered the 7th rank and the white rooks didn't find a way to join the attack. The game reached a RRN vs RRB position, black was a pawn up with a weak pawn structure but its pieces kept the pawns protected. Evals came down, Stockfish gave a rook for a knight and pawn and the game was adjudicated.

In game 46 the engines repeated the reverse for 10 plies, then diverged but transposed back so the real divergence was after 31 plies. Leela had the queen forward on the queen side, it decided not to take the white a pawn immediately, instead Leela attacked the center. In a very long PV agreement the engines cleared the center pawns, Stockfish's eval increased very slowly. Then in a series of exchanges in yet another PV agreement the engines reduced to a queen ending with white a pawn up.

The ending was very difficult to analyze. For the viewers and me the real story was that Stockfish was confident it could win, its eval kept increasing with each move, while Leela felt safe and its eval even drifted down slowly. This went on for almost 30 (!!!) moves, with endless checks from both engines. Leela of course avoided a queen exchange, Stockfish's plan was to walk with its king to the queen side and attack the black a pawn. By move 66 the white king made it to the queen side, still on the back rank. Stockfish already saw a tablebase win according to its eval, Leela's eval was still under 1 (!!!!!).

The white king moved up the board and Leela's eval finally started to react. Stockfish did not stop to capture the a pawn, the white king continued to move towards the black king, pausing on move 79 to move a pawn and reset the 50 move counter. Stockfish pushed the f pawn forward to the 7th rank, the white king move in front of the pawn to avoid more checks, and finally Stockfish queened on move 97, with mate following. Incredible game !! Stockfish wins the game pair, it leads 9-8.

Games 47-48 started with a sideline of the Kan Sicilian, Marocszy bind, Reti variation, played in high level human chess. In game 47 the engines exchanged a pair of knights after the start and the white queen moved forward. Most pieces were concentrated in the center, after some shuffling the engines opened the c file and exchanged a pair of rooks. After some more shuffling there was a series of exchanges that resulted in a queen ending. Leela was up a pawn but evals were close to 0 and the game was adjudicated. In game 48 the engines exchanged pawns and minor pieces after the start, both engines had a passer on the queen side and both queens move forward. Evals came down as Stockfish captured the black passer and exchanged queens. Leela blocked the white passer, the engines shuffled for a while and then reduced to a RBB vs BRN position. Evals were close to 0, Leela traded the rook for two bishops and the game was adjudicated.

Games 49-50 started with a sideline of the Czech Benoni defense, played in high level human chess. In game 49 the center was blocked after the start and Leela attacked it from the king side. There was a long PV agreement that started with a series of exchanges, the engines opened the f file and both engines created a passer in the center. Leela temporarily gave a rook and attacked the black king, Stockfish gave the rook back and stopped the attack. After stablizing the queen side pawns the engines started to shuffle, evals were stable around 1. Stockfish's eval increased after Leela pushed the h pawn, it jumped over 2 after Leela pushed the g pawn as well and moved its h pawn to h6, trapping a black bishop in the corner. The engines exchanged a pair of rooks and both kings ran to the center, only then Leela's eval increased as well - after move 70 !! In a series of exchanges the engines reduced to a same color bishops ending.

Leela gave the g pawn and the h pawn became an advanced passer. Leela used its bishop to keep the black bishop in the corner, Stockfish had 3 connected passers but no way to support them. The black king had to block the white passer and also to block the white bishop from getting to the vulnerable queen side pawns. Stockfish had to make a choice and it chose to abandon the queen side and capture the central passer. There was a pawn race, both engines queened a pawn. Stockfish was first but Leela queened with check and captured the black bishop in the corner. Leela was on time to stop another black pawn promotion, after Stockfish ran out of checks Leela exchanged queens and won with its remaining passers.

In game 50 there was one pawn exchange on the king side after the start, on move 20 Leela initiated an attack on the blocked center from the king side and Stockfish thought this was an inaccuracy. Evals started to increase in a PV agreement, the engines opened the g file, Stockfish exchanged queens and went a pawn up. Then in another PV agreement there was a series of exchanges and the game reached a RN vs BNN position.

Stockfish moved its rook forward behind the black pawns on the queen side and moved its king and knight forward on the king side. Leela couldn't keep all its pawns and pieces safe and Stockfish captured the black f pawn. After another pawn trade the engines reduced to a R vs NN position, Stockfish with 3 passers. 

Leela captured two white passers with its knights, Stockfish captured the remaining black pawns with its rook. The white a pawn was unstoppable, Stockfish queened and mated. There were two white wins in this game pair, Stockfish leads 10-9.