There's no want of effort; in fact, it's because the players are really keyed up that it's so difficult to grind out a win. Only Gelfand managed to pull it off today, in what at least superficially looks like a pretty incredible game. Mamedyarov played 6.Bc4 against Gelfand's Najdorf and threw the kitchen sink at Black's king. Old wisdom says that the way to meet an attack on the flank is by counter-attacking in the center, and Gelfand did that; still, putting the general wisdom into practice is not as easy as it looks! Gelfand took material when it was offered, and didn't hesitate to sac it back when needed. Altogether it led to a peculiar material imbalance, and by the end of the game (on move 39) Gelfand had six(!) pawns (over)compensating for a rook.
The other games were less dramatic. Radjabov and Kramnik once again produced the least interesting battle, but again, it's not from want of effort. Radjabov tested Kramnik's Lasker Defense in a more mainline system. It wasn't that testing, though: Kramnik simply followed the very nice idea displayed in van Wely-Kasimdzhanov, Khanty-Mansiysk 2010, found eight further moves (quite possibly already done at home, beforehand), and then Rajdabov forced a perpetual.
Kamsky-Topalov was another 6.a4 Najdorf, but rather than playing 6...Nc6 followed by 7...e6 Topalov opted for a more classical Najdorf approach with 6...e5. After 7.Nf3 Be7 8.Bg5 White had a tempo-up version of the 7.Be3 e5 8.Nf3 system we saw in game one of the Mamedyarov-Gelfand match, but Topalov never seemed to be in any serious danger.
That cannot be said for Grischuk, who also drew his game with Black. Aronian was pressing for a long time in a Russian Gruenfeld, and eventually won a pawn. Unfortunately, that only occurred in a rook ending, and as everyone "knows", all rook endings are drawn.
With one classical game left to play, all four matches are still undecided. Kamsky and Gelfand are both up a game, and will advance to the semis with any kind of positive result. Kramnik-Radjabov and Grischuk-Aronian are tied.
Games, with my comments, will be posted later today - stay tuned!