NEW YORK — It was an ungainly site, Roger Federer skidding and falling to the ground, helpless on the court.

It lasted only a moment, though, and despite that and a few other scares Saturday, the world's top player advanced to the fourth round of the U.S. Open.

Federer defeated No. 31 Lleyton Hewitt 4-6, 6-3, 7-5, 6-4 to extend his winning streak to 37 matches at Flushing Meadows — and 14 straight over his Aussie rival, the 2001 U.S. Open champion.

Perfect it was not.

Federer was serving, ahead 4-2 and 40-love in the first set, when he proceeded to spray shots all over Arthur Ashe Court, losing five straight points on the way toward dropping the first set. He committed 23 unforced errors in the first.

Even when he looked to be cruising to the victory in the closing set, up 5-2 and getting ready to serve out the match, he got broken. All that did was extend the match a few more minutes, though it did nothing to quash the notion that Federer was not on top of his game on this day.

Hewitt is one of the few players out there who knows what it's like to beat Federer — regularly.

He won eight of the first 10 meetings in this series, but that was years ago, before Federer started winning Grand Slam tournaments with regularity, and before Hewitt started enduring hip problems that dropped him out of the top 100 before his more recent resurgence.

Federer is trying to become the first player to win six straight titles in New York since Bill Tilden in the 1920s. His next match will come against the winner of a later match between American James Blake and Tommy Roberdo of Spain.

In another early match Saturday, 15th-seeded Radek Stepanek defeated 23rd-seeded Philipp Kohlschreiber to push the top 16 seeded players to 34-0 in the tournament. This is the first time in the 41-year history of the Open era that all top-16 men have advanced to the third round of a Grand Slam.

Following Federer-Hewitt into Arthur Ashe Stadium were No. 29 Maria Sharapova and 17-year-old American Melanie Oudin, who advanced with an upset of fourth-seeded Elene Dementieva.

Oudin is seeking to match her fourth-round appearance at Wimbledon this summer, when she became the youngest American to go that far at the All England Club since Jennifer Capriati in 1993.

Also scheduled was John Isner vs. No. 5 Andy Roddick in an All-American matchup and 22nd-seeded Sam Querrey, another American, against No. 12 Robin Soderling, who defeated Rafael Nadal at the French Open before falling to Federer in the final.