After 8 games Stockfish is leading 2-0 with 6 draws. I wasn't expecting such a start, 2 decisive games in the first 8 and both Stockfish wins. After Houdini's strong performance in the rapid tournament I was expecting it be a tougher opponent, but perhaps in long time controls it hasn't improved a lot since stage 3.
This season Jeroen Noonen is in charge of the openings (Cato is still to blame, there's always a reason). The first four openings we've seen were all exciting and full of chances for both sides. We can hope that this trend continues, with as few boring draws as possible.
The engines have 3 hours each to think with increment but the games are usually shorter than 6 hours. We already saw this happen in the season 8 superfinal. At this level the engines recognize draws very quickly, evals converge to 0 and as a result we get short draws.
In the first game Stockfish sacrificed a knight and was sure it could get a 3-fold repetition. Houdini decided not to repeat, and instead traded its queen to get a Q vs RBN position. Stockfish's eval fell to 0 and Houdini's eval eventually did the same for a first draw.
In the reverse game Stockfish played a different defensive line and equalized very quickly. Houdini thought it was a little behind, perhaps because its king was a little exposed. Stockfish gave a rook for a bishop and pawns but the evals remained close to 0. The game ended in a 3-fold repetition.
Game 3 started with a small eval advantage for Stockfish but it didn't last long. There were many exchanges after the opening and by move 31 only RB vs RB remained. Houdini had an extra pawn but no passers. The game ended in a draw, as expected with opposite color bishops.
Houdini kept its advantage longer in the reverse game. It also kept its pieces and developed threats on Stockfish's king. The eval peaked at 1 (0.5 for Stockfish) but got no further. Stockfish hid its king behind a white pawn, and after sacrificing two pawns got a counterattack on Houdini's king. After massive exchanges only RB vs RB remained, and 25 moves later the engines agreed to a draw.
Game 5 started from move 18 of a well known Zaitsev line of the Ruy Lopez (deeply studied in Karpov-Kasparov in their world championship matches). Stockfish sacrificed a pawn on the queen side while shattering the king side pawns and exposing Houdini's king.
The evals rose to about 1 as the engines playes a series of exchanges, at the end of which Stockfish was up a rook for a knight and pawn. Houdini's king was not safe but had less pieces to worry about. Was the material enough for a white win?
Houdini managed to capture the white king side pawns leaving Stockfish with only one pawn in a QRN vs QBN position, and then Stockfish saw the win. After exchanging queens it could block the black king with the rook, allowing its own king to march to the center and eventually capturing the black pawns. After a few moves Houdini saw it was lost as well. First decisive game of the match, can Houdini win the reverse?
In the reverse game 6 Stockfish repeated Houdini's first move but surprisingly did not see its own attacking reply in the PV. Houdini continuation was less sharp and Stockfish shut down the attack very quickly. After several exchanges Stockfish created a queen side passer and Houdini countered with a rook sacrifice and perpetual check. Sadly it did not repeat and went on for 2.5 hours. Stockfish 1-0 after the third opening pair.
Game 7 started with another long book, 14 moves in a King's Indian opening. Stockfish started with an eval advantage of over 0.6, with pawns advancing on the queen side. Houdini's king side pawns were also advanced, threatening white's king while leaving its own king exposed.
Stockfish made a small hole in the pawn lines which allowed its queen to move closer to the black king. Houdini reacted by exchanging queens, evals rising to over 1.
Stockfish then used its queen side advantage, and the fact that the pawns blocked Houdini's pieces, to open up the queen side and create a passer.
Houdini tried to counter with its king side passers but they were easily blocked by the white king and knight. On the other hand Stockfish with a rook on the 7th rank put a lot of pressure on Houdini, the black king was too far for support and the rooks had little room to move. After exchanging a pair of rooks Houdini couldn't stop the pawn from queening without losing material. Second straight win for Stockfish playing white.
In the reverse game Houdini chose a different path, where it had no advantage on the queen side while Stockfish developed a dangerous attack on the king side. However, evals stayed close to 0 except for a few moves Houdini evaluated as a black advantage. In the end Stockfish's king side attack turned into a perpetual check and draw. That's the second opening in a row that Stockfish uses the opening advantage to find a win while Houdini plays with more caution and gets a drawn game.