Prior to this round Fabiano Caruana was 0-3 against Hikaru Nakamura in decisive games played with a classical time control, but that didn't stop the golden boy of the Sinquefield Cup. He outplayed his opponent with the black pieces, and while he could have won a little more easily it was still a convincing victory overall, and he now enjoys a remarkable 5-0 score at the halfway point.
Two other players won today, and share second place. Magnus Carlsen slowly ground out a win in a rook ending against Levon Aronian (winning, like Caruana, with Black) while Veselin Topalov won on the white side of a Najdorf against Maxime Vachier-Lagrave. Both players looked very good in winning, and as both Carlsen and Topalov are very dangerous once their confidence levels go up it's too soon to hand first prize to Caruana. On the other hand, Caruana will have White against both players in the second cycle, making it that much more difficult for them to catch up.
This is especially so with tomorrow's rest day, which might serve to break Caruana's rhythm a bit. So far, however, this is one of the great starts in tournament chess history, going 5-0 against the world's #1 and #2 (former #2 now) and three other players in the top ten.
Round 6 pairings (Tuesday): Nakamura (1.5) - Aronian (1.5), Caruana (5) - Topalov (2.5), Carlsen (2.5) - Vachier-Lagrave (2)
UPDATE: Games here. I've annotated Nakamura-Caruana in some detail and offered a brief explanatory note at the end of Topalov vs. Vachier-Lagrave.