Flushing Meadows, NY -- (Sports Network) - Reigning four-time champion Roger Federer eased into the fourth round of the 2008 U.S. Open on Sunday, defeating Radek Stepanek in straight sets.

The second-seeded Swiss won the match 6-3, 6-3, 6-2, extending his tournament winning streak to 30 straight victories. Federer, a 12-time grand slam champion, hasn't lost here since the fourth round back in 2003.

Federer had been ranked No. 1 in the world since February 2004 before being supplanted at the top by reigning Wimbledon, four-time French Open and Olympic titlist Rafael Nadal on August 18.

The last set of the day saw Serbia's Novak Djokovic, the third seed, bounced back from a first set loss to knock off Marin Cilic of Croatia, 6-7 (7-9), 7-5, 6-4, 7-6 (7-0).

Meanwhile, fifth-seeded Nikolay Davydenko was a third-round winner in straight sets Sunday.

The Russian star defeated his fellow countryman and 26th-seeded Dmitry Tursunov 6-2, 7-6 (7-4), 6-3 at the USTA Billie Jean King National Tennis Center.

Davydenko, seeking a third straight trip to the U.S. Open semifinals, advances to the fourth round.

Andy Roddick, the 2003 title winner here, did not experience the same trouble he had in the second round of the tournament, as the No. 8 seed handled 31st- ranked Andreas Seppi of Italy 6-2, 7-5, 7-6 (7-4).

Eleventh-seeded Chilean Fernando Gonzalez defeated unseeded Finn Jarkko Nieminen 7-5, 6-4, 6-7 (3-7), 6-1.

In a highly anticipated third-round matchup, 15th-seeded Spaniard Tommy Robredo looked impressive in disposing of France's Jo-Wilfried Tsonga, the 19th-seed, 7-6 (7-2), 6-2, 6-3. Tsonga looked sloppy for most of the match, committing 63 unforced errors compared to just 17 for Robredo in his early exit.

In other matches, unseeded Gilles Muller of Luxembourg upset 18th-seeded Spaniard Nicolas Almagro 6-7 (3-7), 3-6, 7-6 (7-5), 7-6 (8-6), 7-5, and another highly-ranked player, 13th-seeded Fernando Verdasco of Spain, was quickly ousted by Russia's Igor Andreev 6-2, 6-4, 6-4.