NEW YORK — If Rafael Nadal was hurting, it didn't show. If he was vulnerable, it didn't last.

Running into an opponent who could match him fist pump for fist pump, but not quite groundstroke for groundstroke, Nadal got stronger as the match went on in a hard-earned, fourth-round win Tuesday night at the U.S. Open.

It was a 6-7 (3), 6-3, 6-1, 6-3 victory over No. 13 Gael Monfils, a Frenchman who feeds off the crowd in New York and has the talent, but not the stamina yet, to put on a great show against the world's best.

"An important match for the confidence," Nadal said.

The first two sets were some of the most entertaining tennis of the tournament, full of stinging forehands and squeakily precise footwork. And when Monfils started asking for more noise on set point in the first — while Nadal was getting ready to serve — it brought some edginess to a match that could have been routine.

"Against Rafa, I have to improve physically, get stronger," Monfils said. "Maybe I'm good for an hour and 30 minutes. Now I have to improve to be good for maybe two hours and a half."

Before he ran out of gas, Monfils got his set. But Nadal won the match and there were no injury timeouts or signs that his stomach muscles or his knees — the two problem points this week and this summer — were any problem.

He did have a patch on his abdomen, shown off when he changed shirts at the end of the match, then got an unwanted kiss from a male fan who ran onto the court from the stands. The fan was arrested and the U.S. Open vowed to review its security procedures.

Nadal said he didn't have a problem with it.

"He said 'I love you,' and he kissed me," Nadal said.

Besides that security breach, it was a good-news day for the third-seeded Spaniard. He took the court a few hours after Britain's favorite, No. 2 Andy Murray, put in a lackluster effort in a 7-5, 6-2, 6-2 upset loss to No. 16 Marin Cilic — a setback Murray called the most disappointing of his career.

The right-hander, who hits a two-handed backhand, said his left wrist was hurting — he could be seen grabbing it and wincing in pain during a changeover — but didn't blame that for his loss.

"Sometimes, if you play badly or whatever, you don't find a way back into the match," Murray said. "Sometimes that can happen. I don't think I'm perfect."

The Murray loss leaves Nadal as the highest seeded player on that side of the draw, and also means he'll jump ahead of Murray when the next rankings are released. After missing about two months, including Wimbledon, to rest his sore knees, Nadal is getting stronger with each match.

"The knees are perfect," he said. "That's very important for me."

He explained his preference not to discuss the stomach, however, by saying he didn't want his future opponents to know his exact status.

"After the tournament, if I have something, I'm going to say" then, he said.

Which means we might have to wait awhile.

The prospect of the next Rafa-Roger (Federer) final at a Grand Slam tournament — tennis fans have had to wait more than seven months, since the Australian Open — is getting more likely.

Federer plays his quarterfinal today against Robin Soderling — the 12th-seeded Swede who knocked Nadal out of the French Open, then lost to Federer in the final.

Nadal's next match will be against No. 11 Fernando Gonzalez, who upset No. 7 Jo-Wilfried Tsonga, 3-6, 6-3, 7-6 (3), 6-4 on Tuesday.

Cilic, meanwhile, will follow his upset over Murray with a match against No. 6 Juan Martin del Potro, a 6-3, 6-3, 6-3 winner over No. 24 Juan Carlos Ferrero.

On the women's side, 17-year-old and unseeded Melanie Oudin tries to keep her dream run going with a quarterfinal match against No. 9 Caroline Wozniacki tonight.

Another unseeded surprise, Kim Clijsters, beat No. 18 Li Na 6-2, 6-4 Tuesday to set up a semifinal meeting against No. 2 Serena Williams, a 6-4, 6-3 winner over 10th-seeded Flavia Pennetta.

"She plays tough. She plays hard," Williams said of Clijsters. "Now it's like a totally different level, because she has absolutely nothing to lose. I think that's when you can play your ultimate best tennis."