Nadal received abdominal treatment from a trainer in the third set of a 7-5, 6-4, 6-4 victory over 32nd-ranked countryman Nicolas Almagro and his reluctance to speak about it after the match added more fitness worries.

"I don't want to talk about injuries," Nadal said. "I try my best every day. I won the match in three sets."

Pressed on whether or not the treatment helped him in the third set, the six-time Grand Slam winner replied, "I won 6-4. You can see I didn't serve very fast but I think I played better from the baseline than before."

Nadal called the trainer after breaking to pull within 2-1 in the third set.

After laying on his back on the concrete for treatment of what could be a flare-up of a muscle strain suffered last month at Cincinnati, Nadal held serve, broke for a 3-2 edge and served out to win despite discomfort.

"I feel it a little bit now but I will try to recover in time for my next match," Nadal said moments after the final point.

Nadal, whose best US Open run came into last year's semi-finals, will play for a quarter-final berth against French 13th seed Gael Monfils, who was ahead 6-3, 6-4, 1-0 when Argentina's Jose Acasuso retired with left knee pain.

"I have to come out ready for the next match," Nadal said. "For today it was enough but for sure I can do better."

Nadal, the reigning Australian Open champion, is coming off a two-month layoff due to knee tendinitis, a break that kept him from defending his 2008 Wimbledon crown.

"I just plan to keep on improving my tennis and keep playing this tournament because it's special to me," Nadal said. "It's special motivation when you try to come back and play your best as fast as possible."

Nadal and Almagro exchanged seven breaks in a first-set groundstroke clinic, but Nadal ended a run of four consecutive breaks by holding to end the set, then broke and held in the last two games of the second set to take charge.

"It was a difficult match," Nadal said. "He started with a break, I have break back. This kind of match is important to win."

British second seed Andy Murray, last year's US Open runner-up, was set for the feature night match at Arthur Ashe Stadium against 195th-ranked US wildcard Taylor Dent, back after missing two years due to major back surgery.

The 22-year-old Scotsman is trying to become the first British man to win a Grand Slam title since Fred Perry in 1936. More

For the first time in Grand Slam history, all 16 top men's seeds reached the third round, none falling until US number five Andy Roddick on Saturday. No more than 13 of the top 16 have ever reached the fourth round at the US Open.

A sore right knee forced French ninth seed Gilles Simon to retire from his third-round match against Juan Carlos Ferrero, the 24th-seeded Spaniard leading 1-6, 6-4, 7-6 (7/5), 1-0 when Simon called it quits.

Ferrero, the 2003 French Open champion and 2003 US Open runner-up who won his first title in six years at Casablanca in April, will face Argentine sixth seed Juan Martin del Potro for a last-eight berth.

Del Potro lost six games in a row at one stage but fired 20 aces in beating Austrian Daniel Koellerer 6-1, 3-6, 6-3, 6-3.

No Frenchman has won a Grand Slam singles crown since Yannick Noah at the 1983 French Open, but seventh seed Jo-Wilfried Tsonga, last year's Australian Open runner-up, and Monfils are still in the hunt to end the drought.

Tsonga ousted compatriot Julien Benneteau 7-6 (7/4), 6-2, 6-4 to book a date with French Open semi-finalist and 2007 Australian Open runner-up Fernando Gonzalez, the Chilean 11th seed who beat Czech Tomas Berdych 7-5, 6-4, 6-4.