After 62 games Stockfish leads 10-4 with 48 draws. Houdini, with another win, is showing that it can beat Stockfish in some positions. It is just that Stockfish beats Houdini more often, it is +6 with 38 games to go in the match.
The opening for game 55 gave white a rook advantage and a trapped knight. The starting eval was in white's favor but not overwhelming, and as the game progressed evals went down a bit. A series of exchanges left RRB vs RBB on the board, a rook pair vs a bishop pair, can Houdini hold?
Evals started to climb faster, Stockfish was getting sure it would win. To me it seemed the bishop and rook dance continued as before, but now Stockfish was able to get its queen side pawns moving, capture a pawn and create a passer. From there the path to a win was clearer. Slow and beautiful RB vs BB endgame win for Stockfish.
Game 56 started with the same material advantage for white, and evals started at 1 and went down gradually. This time when most pieces were exchanged Stockfish got the material back and Houdini's advantage was a bishop pair in a RBB vs RBN position. After exchanging bishops Houdini was able to capture a pawn and create a passer, still only good for a draw. Another demonstration of Stockfish's power in an imbalanced opening.
Game 57 started with the center pawns locked and evals close to 0. The engines mostly shuffled their pieces, exchanging all the knights and stablizing their pawns. After a few pawn exchanges the engines exchanged all rooks and Houdini's queen infiltrated behind the white pawns, leading to a perpetual check draw.
In game 58 it was the bishops that were exchanged off, and from move 20 the engines started to shuffle pieces with a few pawn moves to complete the pawns lines.
After a long shuffle there were a few pawn moves and the engines continued to shuffle. Stockfish's eval was at 0, but Houdini's remained positive. When Houdini saw the 50 move rule approaching its eval gradually decreased, reaching 0 and even negative values (as a result the game was not adjudicated). To avoid a draw Houdini exchanged the knights and opened the h-file. The game passed 100 moves and the engines had under 2 minutes to play, a blitz ! Houdini used the open file to exchange a pair of rooks and suddenly it saw something in the position and its eval started to jump.
Houdini combined threats against the black pawns and a potential mate with the queen on b7, Stockfish defended as much as it could but Houdini found the winning moves. It forced the black pieces back, then brought the queen and rook to the h file forcing a rook exchange. After that the a7 pawn fell and Houdini's pawn on a6 was too strong. All this playing on increments. Houdini proved again it is better in shorter games, recall it won the rapid tournament.
In game 59 Houdini sent a bishop deep into white territory where it got trapped for 8 moves while Stockfish was unable to capture it. The bishop managed to get out of the trap unharmed. After exchanging queens only RBN vs RBN remained on move 27. Evals fell gradually to 0 as the engines exchanged a few pawns, and the game ended in a 3-fold check repetition.
Game 60 reached a rook ending by move 27, evals were at zero a few moves later. The rooks continued to capture pawns until the game reached a tablebase draw.
After the opening in game 61 Stockfish was a pawn up with a queen side passer while Houdini had a supported passer in the center. Stockfish used its knights to block the central black pawns, and together with its own pawns formed a line across the board and started to shuffle pieces behind it. Houdini's line was broken, its pieces defended the entry points dynamically. The standoff continued until reaching a 50-move draw.
In the reverse game Stockfish's eval dropped to 0 immediately. After exchanging all the rooks the black queen moved forward and threatened Houdini's king on the back rank. Houdini was forced to counter with a perpetual check draw.