Nadal appeared to be back to his healthy, hustling self during a 6-7 (3), 6-3, 6-1, 6-3 victory over No. 13-seeded Gael Monfils today, continuing his bid to complete a career Grand Slam of four majors.

"Well, very happy, no? After the last two matches I played, I had a few problems, but, anyway, I am here. I am in the quarterfinals for the third time," Nadal said.

The Spaniard moved well against Monfils, covering the court during their many extended exchanges.

It was a high-quality, high-speed contest over the first two sets, with both men -- considered by many to be the best pure athletes on tour -- displaying tireless defence and powerful offense.

"Unbelievable. Sometimes you think you won the point and the ball is coming back another time and another time and another time," Nadal said.

Perhaps, but it was Monfils who began to tire late in the second set, sometimes leaning over with a hand on his knee to rest and gasp for air between points.

"Didn't have the miles in my legs," Monfils explained.

Nadal, meanwhile, showed no significant signs of problems from either the sore knees that kept him off tour for all of June and July -- forcing him to skip a title defence at Wimbledon -- or the abdominal muscle issue that flared up during his third-round victory over Nicolas Almagro.

"His defence was very, very strong today," Monfils said.

Nadal did have a small bandage on his stomach again today, and he did seem to serve conservatively, with no aces and no double faults.

"I am improving every day. After a month and a half or two months of (not playing), it is not easy coming back," Nadal said.

"The knees are very good, so very happy for that."

He is assured of overtaking Andy Murray in next week's ATP rankings, and a victory over No. 11 Fernando Gonzalez in the quarterfinals would guarantee Nadal a return to No. 2.

Nadal was No. 1 or 2 from July 2005 until three weeks ago, when he dropped to No. 3. But Murray's loss earlier today, coupled with Nadal's victory, means they'll switch places.

There is still a chance Novak Djokovic could pass both of them by winning the title.

When Nadal finished beating Monfils, a spectator ran out of the stands and approached Nadal on the sideline before security guards intervened.

It wasn't nearly as scary an episode as occurred during the French Open final this year, when a man went up to Roger Federer during the match and tried to place a hat on his head.