NEW YORK — Roger Federer stayed perfect at this U.S. Open, beating Radek Stepanek 6-3, 6-3, 6-2 Sunday and breezing into the round of 16.

To win his first Grand Slam of the season, the No. 2-ranked Federer needs to take his fifth straight title at Flushing Meadows. He has not lost a set in three matches, and raised his right fist after zinging a final, winning forehand down the line.

"I guess it's just nice to play well. Pretty simple," Federer said. "No complaints."

Second-seeded Jelena Jankovic won her sort of split decision, beating No. 21 Caroline Wozniacki 3-6, 6-2, 6-1 to reach the quarterfinals.

Jankovic went into her trademark splits early in the third set reaching for a shot. She lost the point and stayed splayed on the court, and fans wondered whether she'd hurt herself.

But Jankovic bounced up and easily bounded around the court, quickly finishing off her 18-year-old Danish opponent.

"My mom told me that I am losing a little bit of weight," Jankovic said. "I'm actually not losing weight, I'm just getting firmer, you know?"

Andy Roddick and Novak Djokovic were set to play third-round matches later. Olympic champion Elena Dementieva faced Li Na at night in the fourth round.

Fifth-seeded Nikolay Davydenko advanced, defeating Dmitry Tursunov 6-2, 7-6 (3), 6-3. Gilles Muller, the only qualifier left on the men's side, rallied past No. 18 Nicolas Almagro 6-7 (3), 3-6, 7-6 (5), 7-6 (6), 7-5.

Almagro provided the day's comic relief midway through his match.

Thinking his shot had missed, Almagro grabbed a towel near the back wall to wipe his face. He then realized the ball was still in play and tried to recover, but lost the point. Muller beat Rafael Nadal at Wimbledon in 2005 and overcame a two-set deficit against Tommy Haas in his previous match.

Stepanek is among nine players to beat Federer this season, sweeping a pair of tiebreakers at Rome in May. But at 29 years old and seeded 28th, the oldest player left in the men's draw was no match for a player who seems to be on a mission.

Federer got broken for the only time in the first set, then broke back at love and held to win. In the third set, he double-faulted twice before winning his final 11 points on serve.

Federer won a gold medal in doubles at the Olympics, but lost in singles.

"It was one of my goals coming into this season, next to trying to win Wimbledon. I missed that one. Oh, well," he said, drawing laughs from crowd.

Jankovic has never reached the final at a major event. She's got an outstanding chance to go far this time — the Williams sisters are in the opposite bracket, No. 1 Ana Ivanovic and No. 3 Svetlana Kuznetsova have lost and 2007 Open champion Justine Henin has retired.

Jankovic next plays the winner of the match between No. 12 Marion Bartoli and No. 29 Sybille Bammer. Along with a trophy, Jankovic is in the chase to wind up as the world No. 1.

"It's a goal, but I want to win a Grand Slam," she said.

Jankovic repeated her victory over Wozniacki from Wimbledon, where the Serbian star also rallied after losing the first set.

Wozniacki started strong, hitting the ball harder and playing much more aggressively than Jankovic. The scoreboards at Arthur Ashe Stadium malfunctioned in the second set — even without them, it was clear Jankovic was in trouble.

By the time the scoreboards came back on, Jankovic was on her way to winning the first game of the final set.

"I just got a bit more defensive. I felt a little bit tired. she took advantage of that," Wozniacki said. "She was just better than me today."

Jankovic won without a lot of her typical drama. There was a slight rift in the first set when she seemed to have more trouble with the officials than the wind. Jankovic disputed two line calls on the same point, then decided against calling for a replay challenge.

"Lose it. Who cares?" she told the chair umpire.