NEW YORK (AP) ― 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.

She and Azarenka described the level of play as being worthy of a Grand Slam semifinal, rather than the Week 1 matchup that it was. But they wound up being drawn to meet this early because Williams is seeded only 28th, a reflection of her lower-than-usual ranking as a result of nearly a year’s absence from the tour because of a series of health scares.

The 29-year-old American already owns 13 major championships, including three at the U.S. Open, which is why, when she was asked how she can play so well now, Williams replied: “I mean, I was a pretty good player before. So just trying to get back into that rhythm and feel it again.”

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 on Saturday night.