I'll catch up with rounds 1-3 either tomorrow or on Sunday, the rest day. For now, round 4:
The 2021 Sinquefield Cup has seen one exciting round after another. Not only has there been a shockingly high percentage of decisive results (12 out of 20!), but the games have been lively, tactical slugfests with brilliant turns that have even impressed the commentators and the players themselves.
Round 4 was no exception, with four decisive games out of five. The only draw was in the marquee matchup between two of the co-leaders, Fabiano Caruana and Wesley So, and even this game could have finished with a winner. So was in serious trouble, but Caruana's 32.b4 turned out to be a mistake, allowing Black enough counterplay to achieve a draw thanks to the rook's access to c4.
The best game of the day, and probably the most important game from a theoretical perspective, was Leinier Dominguez's victory in a Delayed Poisoned Pawn against Maxime Vachier-Lagrave. Dominguez's novelty, 21.Bd3, doesn't promise anything against a sufficiently prepared player, but even a player of MVL's caliber was unable to solve the problems at the board. Dominguez played perfectly, it seems, with 24.h5, 28.Rd2, 31.Qe1, and 32.Rh6 all being especially incisive. It was a great game by Dominguez, and it let him join Caruana and So in a tie for first with 3/4.
Shakhriyar Mamedyarov won a spectacular game of his own in round 3 and came out of the opening in good shape against Sam Shankland. Had he won, he would have joined the leaders at +2, but in the battle of his two bishops and inferior structure against Shankland's two knights (with rooks on the board as well), the knights reigned supreme. Shankland kept the static features of the position in the forefront, and the bishops never had the chance to show their dynamic potential.
Richard Rapport was Mamedyarov's victim in round 3, though it would be better to say that Rapport's ambition was his undoing in that game. Today he bounced back in good style against Peter Svidler, building a kingside attack that proved unstoppable after 25...g3 (Svidler had to try 25...Be7 26.Nxg4 Kg7). It has been a tough tournament for Svidler, who is currently in last place with just half a point out of the four rounds. He came to St. Louis expecting to do commentary for the Rapid & Blitz tournament and the Sinquefield Cup. But because Alexander Grischuk couldn't make it (ironically, he's doing commentary on the event for Chess24), he wound up playing. He's a great player, but it's hard to play in such a field without proper preparation.
Finally, the previous cellar dweller and (by a significant margin) bottom seed, Dariusz Swiercz, avoided "Audi rings" (four zeros) by defeating Jeffery Xiong. Swiercz was quickly winning on the white side of a Delayed Poisoned Pawn, and although he gave himself far more work than necessary, he did eventually manage to grind out the victory.
The games (with my comments to three of them) are here, and these are the pairings for Saturday's round 5: