Federer's fourth-round match against Monaco didn't get started until nearly midnight, and thanks to his superb play, it was over shortly before 1:15 a.m. Tuesday.

Federer played brilliantly right from the start, taking the first five games - and 20 of the first 25 points - in only 12 minutes. He didn't miss a beat in the second set, hitting four aces in his opening service game and finishing with 14.

Five of Federer's record 16 major championships have come at Flushing Meadows. In the quarterfinals, Federer will face 11th-seeded Jo-Wilfried Tsonga of France, who eliminated No. 8 Mardy Fish of the United States in five sets on Monday.

Tsonga upset Federer in the Wimbledon quarterfinals two months ago, coming all the way back after dropping the first two sets.

"He's a tough player. ... I look forward to that match," Federer said. "If I play as good as I did today, sure I have a chance." Federer compiled a 42-4 edge in winners against Monaco.

The start of their match was delayed because the preceding match in Arthur Ashe Stadium, No. 1 Caroline Wozniacki's 6-7 (6), 7-5, 6-1 victory over No. 15 Svetlana Kuznetsova, lasted 3 hours, 2 minutes.

"You have to be ready," Federer said.

Wozniacki, seeking her first Grand Slam title, trailed by a set and 4-1 in the second before coming back to beat the 2004 U.S. Open champion.

"I knew that I had to do something," said Wozniacki, who faces No. 10 Andrea Petkovic of Germany next. "I had to do something different." She managed to turn things around thanks to a combination of her own increasingly aggressive play and Kuznetsova's increased mistakes. Kuznetsova's 40-20 edge in winners was rendered meaningless by her 78 unforced errors, 52 more than Wozniacki, who reached the US Open quarterfinals for the third year in a row.

Tsonga made it this far in New York for the first time, and unlike Fish, he's already tasted this sort of success.

Tsonga made it to the final of the 2008 Australian Open before losing to Novak Djokovic, and got to the Wimbledon semifinals this year, where he again lost to Djokovic.

On Monday, Djokovic extended his 2011 record to 61-2 by beating No. 22 Alexandr Dolgopolov of Ukraine 7-6 (14), 6-4, 6-2. Their 16-14 tiebreaker in the first set lasted nearly half an hour all on its own, with Djokovic saving four set points and finally converting his sixth when Dolgopolov pushed a forehand long to close a 13-stroke exchange.

Both men called that tiebreaker the key to the match. One tiny piece of evidence: Dolgopolov double-faulted twice in the opening game of the second set to get broken, and Djokovic was on his way.

Asked whether he considered winning that energy- and will-testing tiebreaker to be a physical or mental triumph, Djokovic replied: "Combination of both. But in the end, it was more mental, just to hang in there, try to play right shots at the right time." Djokovic now meets his Serbian Davis Cup teammate and friend Janko Tipsarevic, who is seeded 20th and made it to the first Grand Slam quarterfinal of his career by beating 2003 French champion and U.S. Open runner-up Juan Carlos Ferrero of Spain 7-5, 5-7, 7-5, 6-2 in a match that lasted more than 3½ hours.

"Strange feeling," Djokovic said. "We are professionals. Certainly we both want to win the match when we play against each other. So you kind of forget about friendship. You put that aside." Most players complained about the wind, which gusted at up to 20 mph (32 kph) and kept changing directions, making even serve tosses difficult.

Serena Williams handled those conditions much better than former No. 1 and 2008 French Open champion Ana Ivanovic and beat her 6-3, 6-4 to return to a Grand Slam quarterfinal for the first time in 14 months. She missed about 11 of those with a series of health scares but looks really good so far at the US Open.

Against Ivanovic, Williams hit nine aces overall, only lost serve once, and finished off the match with four consecutive unreturned serves that ranged from 99 to 111 mph.

"I didn't even go for winners at any point," said Williams, who hit only 16. "I just tried to get it over because it was so windy. It was definitely tough." She's seeded only 28th because of all of that time away, but now has won her past 16 matches heading into a quarterfinal against No. 17 Anastasia Pavlyuchenkova of Russia. Pavlyuchenkova got past 2010 French Open champion Francesca Schiavone 5-7, 6-3, 6-4 in a match with 21 double-faults and 16 service breaks in 31 games.

"I'm going to say that I don't want to go out there and enjoy just being on centre court playing against Serena," Pavlyuchenkova said. "I would like to do well, try to fight, and with my effort, I'll try to beat her."