NEW YORK — Disappointed.

That was the word Andy Murray used over and over to describe how he felt about his exit from the U.S. Open.

After all, Murray arrived at Flushing Meadows ranked No. 2, owner of a tour-leading 37 wins on hard courts this season — and quite sure he was prepared to win his first Grand Slam title a year after reaching the U.S. Open final. Instead, he heads home after the fourth round, a 7-5, 6-2, 6-2 loser to No. 16 Marin Cilic of Croatia yesterday.

"I just struggled today. I played poorly," Murray said. "I could have been better in pretty much every part of the game, whether it was mental or serve, forehand, backhand, returns."

And now Murray will slide back to No. 3 or No. 4 in the rankings, a spot behind Rafael Nadal, who will move up by virtue of beating No. 13 Gael Monfils, 6-7 (3), 6-3, 6-1, 6-3, in last night's last match to get to the quarterfinals.

Nadal-Monfils featured two men often cited as the best athletes on tour, and it was a spirited contest early, filled with long points, terrific retrieving, powerful hitting — and animated celebrations by both. Monfils eventually began to show signs of fatigue, leaning over to gasp for air between points, and Nadal took control.

Nadal, trying to complete a career Grand Slam by winning the U.S. Open, will meet No. 11 Fernando Gonzalez of Chile in the semifinals. Gonzalez eliminated No. 7 Jo-Wilfried Tsonga of France, 3-6, 6-3, 7-6 (3), 6-4.

"First thing that I have to try is play my tennis," Gonzalez said. "If I don't play my tennis, I mean, I have no chance."

Murray definitely had his chances. Ahead 5-4, he wasted two set points. By the second set, Murray was grimacing while flexing his left wrist, which he acknowledged afterward had been bothering him for about a week. By the third set, Murray was moping about the court.

"Regardless of my wrist, I lost the match," he said. "I returned poorly. He served well — and that was really the difference."

Murray's loss, at least for one evening, turned into the talk of a tournament that had been generating very little buzz about the men's field. Instead, the focus has been on the women's event, thanks to the surprising emergence of 17-year-old American Melanie Oudin and the quick comeback of former No. 1 Kim Clijsters.

Oudin plays in the first major quarterfinal of her nascent career today and Clijsters already is into the semifinals, where she will face a Williams for the second time this tournament.

Clijsters beat Venus Williams in the fourth round; now she will take on defending champion Serena Williams, who improved to 23-1 in Grand Slam singles matches this season by beating No. 10 Flavia Pennetta of Italy, 6-4, 6-3, last night.

Serena Williams and Clijsters both have 12-match winning streaks at the U.S. Open. Clijsters won the first seven en route to the 2005 title, the last time she played in New York, and now has five this year, the latest a 6-2, 6-4 win against 18th-seeded Li Na of China yesterday.