NEW YORK — Roger Federer once wondered how he was ever going to beat Lleyton Hewitt.

Hewitt dominated the series back when the Australian was in the midst of winning two Grand Slam titles and Federer was still chasing his first major championship.

Hard to remember now that the top-ranked Swiss has won 13 straight matchups.

The two set up another showdown by winning their second-round matches Wednesday at the U.S. Open. Federer beat Simon Greul 6-3, 7-5, 7-5, while Hewitt defeated Juan Ignacio Chela 6-3, 6-3, 6-4.

"On any given day, a former world No. 1, a guy who's won majors, is very, very dangerous," Federer said. "That's why I have to make sure I get into the match quickly, not give him the lead, because we know he's not going to go down without a fight."

The player who has given Federer fits more recently — Rafael Nadal — motored around the court like a guy with two healthy knees Wednesday. He defeated Richard Gasquet 6-2, 6-2, 6-3 in his first Grand Slam action since May 31, when Nadal's 31-match French Open winning streak ended in the fourth round at Roland Garros.

Tendinitis in both knees sidelined Nadal for more than two months. He pulled out of Wimbledon instead of defending his title, saying he couldn't play without thinking about the injury.

Not anymore.

"I don't have pain, so I don't think," Nadal said.

The defending women's champion, Serena Williams, needed just 53 minutes to dispatch 51st-ranked Melinda Czink 6-1, 6-1 in Wednesday's last match.

Her next opponent is Maria Jose Martinez Sanchez, who pushed Williams to three sets in a drama-filled match in the third round of the French Open. Williams insisted the ball went off Martinez Sanchez's arm on a point that helped the Spaniard win the first set; she later accused Martinez Sanchez of cheating.

"She plays a fun game," Williams said Wednesday. "She's a tough cookie."

Venus Williams had wide patches of white tape above and below her left knee, which began bothering her when she struggled through a first-round win Monday. But like Nadal, Williams looked just fine Wednesday, and she easily eliminated Bethanie Mattek-Sands of the United States 6-4, 6-2.

Kim Clijsters was away for two years, not two months, but she's rolling, too. After ending her retirement in August, she continues to play as if she never left. Unseeded, unranked and playing at the U.S. Open for the first time since winning the 2005 title, the 26-year-old Belgian reached the third round by knocking off No. 14-seeded Marion Bartoli 5-7, 6-1, 6-2.

Two fixtures on the men's tour said goodbye to Grand Slam tennis with first-round exits: Marat Safin of Russia and Fabrice Santoro of France, who are retiring at the end of the season.

The 29-year-old Safin, the 2000 U.S. Open champion, lost to Jurgen Melzer of Austria 1-6, 6-4, 6-3, 6-4; the 36-year-old Santoro, appearing in his record 69th major tournament, was beaten by No. 24 Juan Carlos Ferrero of Spain 6-4, 6-3, 6-3.

Former U.S. Open champs Maria Sharapova, Andy Roddick and Svetlana Kuznetsova play today. Dinara Safina returns to the court after nearly becoming the first women's No. 1 seed to lose in the Open's first round Tuesday.