Spanish third seed Nadal, trying to complete a career Grand Slam with a seventh major title, defeated French 13th seed Gael Monfils 6-7 (3/7), 6-3, 6-1, 6-3 to book a spot in the last eight against Chilean Fernando Gonzalez.

"He's a very good player," Nadal said. "If I don't play my best it's going to be impossible."

Nadal showed no sign of pain after a two-month layoff with knee tendinitis, calling his knees "very good," or from a right abdominal muscle injury that was treated during his third-round victory Sunday over countryman Nicolas Almagro.

"I had a few problems but I am in the quarterfinals here for the third time and I'm very happy," Nadal said. "I'm just trying to enjoy my comeback. It's a big result for me. It's much better than what I expected."

A spectator, camera in hand, ran onto the court moments after the match and reached Nadal to give congratulations before security guards pulled him away.

Nearly as shocking as the surprise intruder was Murray's fourth-round exit, the second seed never looking on form as Croatian 16th seed Marin Cilic reached his first Grand Slam quarterfinal with a 7-5, 6-2, 6-2 victory.

"I just struggled," Murray said. "I played poorly. I'm very disappointed. I didn't feel like I played well. I had my chance in the first set and struggled after that."

The 22-year-old who was last year's U.S. Open runner-up failed in his bid to become the first British men's Grand Slam champion since Fred Perry in 1936.

"The way the match went makes it disappointing but I believe I will come back better from it," Murray said. "I will learn from this week like I have most weeks when I've lost and I will come back better and stronger.

"I think I will be a better player next year than this year."

Cilic will play for a semifinal berth against Argentine sixth seed Juan Martin Del Potro, who fired 22 aces and 44 winners to defeat Spanish 24th seed Juan Carlos Ferrero 6-3, 6-3, 6-3.

"I have my confidence in the best moment," Del Potro said. "I've beaten many good players on this surface. I would like to be in the semis or my first final. I have everything to do it."

Gonzalez, the 11th seed, advanced past French seventh seed Jo-Wilfried Tsonga 3-6, 6-3, 7-6 (7/3), 6-4. Gonzalez, trying to become the first South American man to win the U.S. Open in 32 years, is 3-6 lifetime against Nadal.

"I have a lot of confidence," Gonzalez said. "I have to be worrying about myself only and try to play my best tennis."

Gonzalez, the 2007 Australian Open runner-up in his best Slam showing, had not reached the U.S. Open final eight since his 2002 maiden Slam quarterfinal appearance but denied Tsonga on 13-of-14 break-point chances to advance.

Nadal is trying to become just the seventh man to complete the career Grand Slam, joining Roger Federer, Andre Agassi, Fred Perry, Don Budge, Rod Laver and Roy Emerson as the only men to win each of the four major titles.

Monfils won five of the last six points in the tie-break but was undone by 63 unforced errors. Nadal broke twice to claim the second set, three times including the first and last games to take the third.

Nadal and Monfils exchanged breaks in the fourth and fifth games of the last set but the Frenchman double-faulted away a break in the eighth game and Nadal held to win after two hours 45 minutes.

In taking his first victory over a top-three opponent in seven attempts, Cilic fired 10 aces and 35 winners while Murray went 0-for-7 on break-point chances and made 41 unforced errors.

"I played very well and he was missing a lot," Cilic said. "I don't think he was playing his best."

Defeat on the Flushing Meadows hardcourts was especially stinging for the 22-year-old Scotsman, who loves the surface as well as the atmosphere of a New York fortnight in the year's last Grand Slam event.

"I returned poorly. He served well. That felt like the difference," Murray said. "The momentum was with him and I didn't manage to get it back."