After 40 games Stockfish leads 11-7 with 22 draws. Stockfish extended its lead to 4 wins after winning two game pairs, three game pairs were drawn, one of which had two white wins. In the two game pairs Stockfish won it displayed superior endgame skills. In one game pair the two games reached exactly the same RB vs RB endgame, Stockfish managed to draw as black and win as white. In the other Stockfish won a queen ending which Leela seemed to think was a draw for a long time.
Games 31-32 started with a 24-ply line in the Ruy Lopez, Open, classical defense, played at the highest level of human games. There was one pawn exchange and all pieces were on the board. In game 31 after two moves the engines started to play out a long PV agreement, exchanging most pieces and reducing to a RBN vs RBN position on move 21. Leela was a pawn up, its eval went up for a for a few moves but then came down. The engines traded pawns until all pawns were passers, 3 white on the queen side and 2 black on the king side. Evals were low, after exchanging more pieces the game was adjudicated.
In game 32 Leela's first move was different, the black king was vulnerable and Stockfish placed a bishop on h6. The engines exchanged all knights in the center, Leela gave a pawn and both engines created a passer, evals started to increase slowly. Stockfish gave the pawn back, the engines exchanged a pair of rooks and Stockfish replaced the bishop on h6 with a pawn. In a series of exchanges the engines reduced to a queen ending with white a pawn up.
Stockfish used checks to improve its position while keeping its pawns safe. Leela's eval did not increase at first, it jumped when the white king moved forward. Stockfish saw the win in its eval, it forced a queen exchange on move 82, then it queened and mated. Stockfish wins the game pair, it leads 9-6.
Games 33-34 started with an 18-ply line in the Ponziani opening, Jaenisch counter attack. The white king side was exposed with the black queen on h3, black had the advantage. In game 33 Stockfish avoided a queen exchange, it castled long while the white king moved without castling. Leela was slow to develop the queen side, though (negative) evals came down. Stockfish created a passer on the king side and pushed it forward while Leela captured a pawn. Stockfish gave a knight and attacked the white king, Leela lost a rook for the passer and the game reached a B vs R ending. Both engines had two pawns on the queen side and Leela had a fortress. Stockfish wouldn't lower its eval for the draw rule, the game was adjudicated only on move 159. In game 34 Leela castled long, the queens were off early and the white king was not in danger. (negative) Evals came down, Stockfish captured pawns on the king side and gave a rook for a bishop. Leela created a passer on the king side and pushed it forward, Stockfish blocked with a rook. In a RBN vs RRN position Stockfish gave two minors for a rook and captured the black passer. Evals were close to 0 and the game was adjudicated.
Games 35-36 started with a sideline of the Old Indian defense. In game 35 Leela moved its queen forward and captured the b7 pawn, Stockfish tried to trap the white queen. Leela moved a knight forward and captured a second pawn, then gave a rook for a bishop and moved its second knight forward to protect the first knight and to let the queen retreat. Evals came down, there were more exchanges and eventually the engines reduced to a QRB vs QRN position. After exchanging queens the game was adjudicated. In game 36 Stockfish moved its queen forward, it captured two pawns on the queen side and Leela didn't try to stop it. Stockfish exchanged queens, it was two pawns up but its development was slower and its king castled late. Leela moved a rook to the 2nd rank and in a RRB vs RRN position captured one pawn back. Evals came down, the engines continued to exchange pawns and pieces and the game was adjudicated in a rook ending.
Games 37-38 started with a 32-ply line in the Sicilian Dragon, Yugoslav attack. The engines castled in opposite directions, black was a pawn up and the h file was half open. In game 37 Stockfish gave a rook for a bishop early, then it gave another rook for a knight and exchanged queens. On move 29 the game reached a RR vs BN position with black 3 pawns up.
Stockfish tried to block files to restrict the white rooks, Leela gave a pawn and opened the queen side. Stockfish captured all the white pawns but Leela moved its rooks forward and captured pawns. Eventually Leela trapped and captured the black bishop, the game ended in a tablebase win.
In game 38 the engines exchanged bishops and Stockfish moved its queen to h6. The black king was trapped and Leela moved its queen to help it. Stockfish exchanged a pair of knights and regained the pawn, then the queens were exchanged. Stockfish captured another pawn and created a passer on the king side, on move 31 the game reached a RN vs RB position.
Leela blocked the passer with its king. In a long PV agreement the engines exchanged pawns and Stockfish abandoned its passer, though evals continued to increase. On move 51 the remaining pawns were on the queen side, each engine had 3 pawns, the black pawns were all on dark squares and the white pawns on light squares.
Two black pawns were weak and the bishop could not protect them. Stockfish made sure its b3 pawn had enough defenders, slowly it captured black pawns and on move 75 all were captured. Stockfish had two pawns, Leela lost material and the game ended in a tablebase win. There were two white wins in this game pair, Stockfish leads 10-7.
Games 39-40 started with a 20-ply line in the Semi Slav, accelerated Meran variation. Both queens moved forward on the queen side where a pair of pawns was exchanged. In game 39 the engines played out a long PV agreement, they opened files on the queen side, Stockfish kept its king uncastled. Stockfish gave two pawns and then in a series of exchanges the game reached a RB vs RB position. Leela was two pawns up with no passer and doubled pawns on the queen side. After a while Stockfish managed to captured one of the doubled pawns and formed a strong bishop-pawn pair on the queen side. Evals slowly came down while the engines shuffled, Leela couldn't find a way to improve. There were a few pawn exchanges, Leela was slow in lowering its eval, the game was adjudicated on move 112.
In game 40 the engines repeated game 39 for 72 plys, with transpositions and in some cases slightly different moves. The game reached a RB vs RB position with white a pawn up, and with such a long repeat and evals stable and not high it was obvious the game would have the same outcome. On move 53 Stockfish's eval jumped, but after a pawn exchange a few moves later it came back down. On move 73 Stockfish's eval jumped again, this time it kept increasing. Leela's eval remained low for more than 15 moves, the white king managed to move forward through the center and Stockfish created a passer on move 90.
Leela's eval started to increase while Stockfish already saw the win. Leela captured the two pawns on the king side but Stockfish threatened mate and Leela gave a rook for a bishop. The game ended in a tablebase win with one last white pawn. Stockfish wins the game pair, it leads 11-7.
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