After 20 games Stockfish leads 5-2 with 13 draws. There were 2 double draws and the first game pair with two white wins. Stockfish won two game pairs and extended its lead. In both cases the win was a long and slow endgame.
Games 11-12 started with a 24-ply line in the French, MacCutcheon, Lasker variation. The white king moved forward without castling and all pawns were on the board. In game 11 the engines blocked the center, Stockfish castled long and Leela captured a pawn on the king side. The pawns stabilized and the engines started to shuffle, with evals coming down gradually. On move 79 Leela moved a pawn to reset the 50-move counter, Stockfish captured the pawn. After a series of exchanges the game reached a QRR vs QRR position, the game ended 20 moves later in a 3-fold repetition. In game 12 Stockfish opened the center and king side, Leela kept its king in the center uncastled. Evals came down and the game reached a QRN vs QRN position. Leela captured the white pawns on the queen side, it had a passer there. Both engines gave checks with their queens, then in a series of exchanges reduced to a rook ending and the game was adjudicated.
Games 13-14 started with the unusual 1. e3 f5 Van't Kruijs opening. In game 13 the engines opened the center in a long PV agreement. The game reached a RBB vs RBN position on move 27 and evals started to come down. The engines reduced to a same color bishop ending, it took another 60 moves before Leela lowered its eval enough for the draw rule. In game 14 there were only a few exchanges after the start. Stockfish pushed pawns on the queen side and opened the b file. Leela pushed pawns on the king side and evals came down. The engines cleared most pawns on the king side and exchanged queens, Stockfish captured a pawn and had a passer in the center. The game reached a RRN vs RRN position with low evals, the game was adjudicated after a pair of rooks was exchanged.
Games 15-16 started with a Pirc, Austrian attack, played in high level human games. In game 15 Leela pushed pawns on the queen side, the engines exchanged a few minor pieces and then started to shuffle. On move 39 the engines exchanged a pair of bishops and continued to shuffle. The white king walked to the center and Stockfish opened the queen side on move 63. The white king became exposed, after the engines avoided exchanges Stockfish moved its queen forward and gave checks. Stockfish gave a rook and cleared the pawns on the queen side. The game reached a QRR vs QR position, black was two pawns up but the its king's safety was worth the material. The game was adjudicated 20 moves later after the engines exchanged queens.
In game 16 Stockfish pushed pawns in the center and its eval stared to increase early. On move 12 Leela predicted the next 18 plys and its eval stayed constant. Stockfish opened the h file and castled long while Leela captured two pawns and opened the center. Stockfish surprised Leela on move 22, after more than 10 minutes thinking Leela's eval dropped almost to 0. The engines continued with a PV agreement of 25 plys. Leela's eval jumped over 2 after another long think, Stockfish chased the black king to the center and then reduced to a RR vs RN position. Stockfish continued to chase the black king until it was trapped on the a file.
The endgame was very slow. On move 51 Leela captured the white f pawn and created connected passers, but this allowed Stockfish to move a second rook forward on the queen side. A mate threat forced Leela to exchange a pair of rooks. Stockfish created a passer on the queen side, queened and mated. Stockfish wins the game pair, it leads 3-1.
Games 17-18 started with a 20-ply line in the Nimzo-Indian defense, Kmoch variation. In game 17 Stockfish pushed pawns on the queen side, opened the a file and created connected passers. The engines exchanged a pair of rooks, Leela had a passer in the center and evals gradually increased. Leela's eval jumped over 2 despite the advancing black passers that trapped a bishop on the back rank.
Leela blocked the black passers with a knight on c3 and then exchanged queens. Leela pushed pawns on the king side and Stockfish gave a pawn to break the pawn line. After exchanging a pair of knights the game reached a RBB vs RBN position, Leela brought its king to c3 to block the black passers. The trapped white bishop was still trapped but both evals were high at this point.
Leela captured both black passers and connected a second passer in the center. The trapped bishop became free to move, Leela used it to pin the knight and then capture it. The game ended in a tablebase win.
In game 18 Stockfish immediately played a4 after book to stop any advancing pawns on the queen side. Stockfish pushed pawns on the king side and gave a pawn. Leela was not impressed but was surprised by Stockfish's move 22.
Leela expected an exchange on f5, instead Stockfish exposed the black king, Leela thought for 10 minutes and its eval jumped to 1.5. The black king moved toward the center and Stockfish opened the e file. The white major pieces threatened the black king, in a series of exchanges the engines reduced to a Q vs RN position on move 41.
Stockfish captured pawns and saw the win in its PV. The white king moved forward to support the passers, Leela lost material and the game ended in a tablebase win. There were two white wins in this game pair, Stockfish leads 4-2.
Games 19-20 started with a line in the Caro-Kann advance, Short variation, played in high level human games. In game 19 the center was blocked and the engines exchanged minor pieces. Leela pushed the h pawn forward and Stockfish castled long. Leela opened the h file and the engines exchanged a pair of rooks, Stockfish's eval dropped and the game reached a BB vs BN position. The engines started to shuffle and the game was adjudicated.
In game 20 the engines started similarly, though Stockfish kept its king side pawns back. Leela castled long and Stockfish opened the center, it thought Leela's move 21 was inaccurate and its eval jumped. In a long PV agreement the engines reduced to a Q vs RR position, Leela's eval was still close to 1.
The endgame was slow, Stockfish's eval increased steadily and Leela's eval followed more slowly. Stockfish pushed pawns on the king side and created a passer, then slowly pushed it to the 7th rank. Leela created a passer on the queen side but it was too slow. The white king moved forward, Stockfish lost the passer but captured the pawns in the center and created more passers. Leela lost one of the rooks and the game ended in a tablebase win. Stockfish wins the game pair, it leads 5-2.
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