After 66 games Stockfish leads by 11-5 with 50 draws. Stockfish keeps its +6 lead while Houdini collects another win, with the help of the opening this time.
The opening of game 63 was a sharp position from the Velimirovic line in the Sicillian. When the initial storm quieted down Stockfish was up a pawn with connected passers and an exposed king on the queen side, and Houdini had a passer in the center. Stockfish had an eval advantage that went up to 1, but after exchanging a pair of rooks the evals started to go down. In a QRB vs QRB position Houdini's passer was blocked on the 3rd rank and couldn't advance while Stockfish couldn't safely capture it. The game ended in a 3-fold repetition.
In game 64 the queens were exchanged early, Houdini tried to keep its advantage by giving a rook for a knight and pawns. After a series of exchanges only B vs BN remained and Houdini was 3 pawns up, connected passers on the queen side. As the game progressed it became clear that Stockfish's extra knight was not strong enough to stop the white pawns, but with opposite color bishops Houdini could only reach the 7th file with a pawn. In the end Stockfish could sacrifice its pieces for the remaining pawns to get a pawnless draw.
Game 65 started with an eval advantage for Stockfish that gradually increased to over 1. After a series of exchanges only QRB vs QRB remained in what seemed to be a balanced position.
Material was equal and the pawn structures were identical. However, the white rook was much better placed and it could immediately block the black rook from defending the b pawn. In the long run the black b pawn was doomed. Another weakness was the black king, too far from the action and under threat of a potential back rank mate. To exploit these weaknesses Stockfish exchanged queens and captured the black b pawn.
After exchanging the bishops the white b pawn became too strong and there was no hope of a king side counterplay. Stockfish won a few moves later.
Game 66 started similarly, with an eval advantage for Houdini of about 1. The exchanges started earlier in this game, the result of which was again a QRB vs QRB position.
There were more pawns in this position, material was equal but Houdini had a passer. The black c pawn was hard to defend and the black king was a weakness as before. After Houdini captured the c pawn and exchanged bishops the action was around the white passer. Houdini's evals were already high in the following beautiful position.