NEW YORK - If anyone still harbored any doubts about whether Serena Williams is back at her best, she put on a pretty persuasive performance during the first 17 minutes of her third-round match Saturday at the U.S. Open.

That's how long Williams needed to build a 5-0 lead en route to a 6-1, 7-6 (5) victory at Flushing Meadows over one of the best players the women's field had to offer: fourth-seeded Victoria Azarenka, a Wimbledon semifinalist two months ago.

Listen to Azarenka describe how it felt to stand across the net from Williams during her superb start:

"What's it like? It's painful," said Azarenka, who won eight points in those opening five games. "To have somebody just going at you like that, it's a little bit painful."

Eventually, though, Azarenka straightened her own play out enough to make things interesting in the second set. She erased four match points, broke when Williams served for the match at 5-3, and left Williams saying she wasn't pleased.

"She won the first set very easily, and it was a little bit too easy, so then she got tested - and she needed to be tested," said Williams' mother, Oracene Price.

Williams passed the test, and showed off her fitness, which is much improved from when she lost in the fourth round at Wimbledon. At the end of one point, she did the splits. On another, she raced all the way over to the blue sign near the stands to smack a forehand winner.

In the fourth round, Williams will face former No. 1 Ana Ivanovic of Serbia, the 2008 French Open champion, who is seeded 16th.

"It's exciting, at least for me. I think she's the hottest player out there at the moment. She's been playing so well lately. It is going to be a good challenge," Ivanovic said after ending Sloane Stephens' surprising run by beating the 18-year-old American 6-3, 6-4 in Arthur Ashe Stadium.

Her match against Stephens was followed by top-seeded Novak Djokovic's 6-4, 6-4, 6-2 victory over 39th-ranked Nikolay Davydenko, a two-time semifinalist at the U.S. Open. After improving to 60-2 in 2011, Djokovic delighted the crowd by showing off some dance moves on court while music blared over the stadium loudspeakers.

At the start of the day, the woman who's seeded No. 1, Caroline Wozniacki, took to that same court and eliminated Vania King of the United States 6-2, 6-4. Wozniacki will continue her bid for a first Grand Slam title against 15th-seeded Svetlana Kuznetsova, owner of two major trophies.

Roger Federer moved into the fourth round for the 30th consecutive Grand Slam tournament by overcoming what he called "tricky wind" and a second-set blip to defeat No. 27 Marin Cilic of Croatia 6-3, 4-6, 6-4, 6-2.

Also advancing Saturday were No. 8 Mardy Fish, the top-seeded American, who has yet to drop a set after beating Kevin Anderson of South Africa 6-4, 7-6 (4), 7-6 (3); No. 20 Janko Tipsarevic of Serbia; No. 22 Alexandr Dolgopolov of Ukraine; and 2003 French Open champion and U.S. Open runner-up Juan Carlos Ferrero of Spain.