Roger Federer was made to sweat literally if not metaphorically as he progressed to the fourth round of the US Open with a straight-sets win against Fernando Verdasco.

Federer had not been tested in his first two matches against Donald Young and Bjorn Phau, but former grand slam semi-finalist Verdasco was a step up in class, even if he had lost his four previous meetings with the world number one.

The conditions on Arthur Ashe Stadium were so hot that even the normally ice-cool Federer was dripping, but one break in each set proved enough in a 6-3 6-4 6-4 victory.

The top seed revealed afterwards that he was thinking of last year's semi-final defeat by Novak Djokovic when he served for the match.

Unlike on that occasion, though, where he held two match points but was broken, Federer had no problems completing the job.

He said: "Coming out during the day, when I walked on, it reminded me of the ovation I got from that particular match, and then when I got up to serve for it, I think it was the same end that it was against Novak. I just tried to serve this one through.

"It doesn't matter whether you think about it or not. It was funny that I thought about it in a third-round match. But I'm happy that I survived it. I played a really good last game so I'm happy."

Federer also responded to reports today that Rafael Nadal, who is missing the tournament because of a knee injury, may not play again for the rest of the year.

"It's obviously not great news but one that was a possibility," Federer said. "So I'm not shocked about the news. I'm still hopeful that he'll be okay for the rest of the year."

Andy Murray survived a very tough encounter with Feliciano Lopez to reach the fourth round.

The third seed had beaten the Spaniard in all of their six previous meetings, mostly very comfortably, but there was nothing comfortable about this encounter as Murray eventually came through 7-6 (7/5) 7-6 (7/5) 4-6 7-6 (7/4) after three hours and 53 minutes.

Slovakia's Martin Klizan followed up his shock victory over fifth seed Jo-Wilfried Tsonga by beating Jeremy Chardy to reach the fourth round.

The 23-year-old caused the shock of the men's draw so far on Thursday and he was at it again on Saturday, overcoming 32nd seed Chardy from France in straight sets, 6-4 6-4 6-4.

In the last 16, Klizan will play Croatian 12th seed Marin Cilic, who was an impressive four-set winner over Japan's 17th seed Kei Nishikori.

Cilic has had a lean couple of years after rocketing up the rankings as a teenager, but has been quietly coming back into some form this season.

Nicolas Almagro ended the hopes of US teenager Jack Sock, but not without a struggle, as the 11th seed came through 7-6 (7/3) 6-7 (4/7) 7-6 (7/2) 6-1.

Sock, 19, was looking to become the first American teenager since Andy Roddick 11 years ago to reach the fourth round and took great encouragement from his performance.

"I feel good about my game and my physicality and my overall professionalism and the mental side," Sock said. "Everything in my game feels better now and definitely closer to the next level."

Canada's Milos Raonic is the brightest of the game's rising stars and he showed what a danger he could be to third seed Andy Murray in the next round with a crushing 6-3 6-0 7-6 (7/3) victory over American wild card James Blake.

While Murray took almost four hours to see off Feliciano Lopez in searing heat, Raonic needed only an hour and 48 minutes to win with a display of huge serving and powerful groundstrokes.

Sixth seed Tomas Berdych had a tougher time against in-form American Sam Querrey but the Czech recovered from a set and a break down to win 6-7 (6/8) 6-4 6-3 6-2 and set up a last-16 clash against Almagro.