Novak Djokovic joined four-time defending champion Roger Federer in the fourth round of the US Open with victory over Marin Cilic at Flushing Meadows.

Federer cruised thorough with a 6-3, 6-3, 6-2 victory over Czech Radek Stepanek yesterday and the second seed was joined in the round of 16 by 2003 winner Andy Roddick, who moved on with a 6-2, 7-5, 7-6 victory over Italian Andreas Seppi. Third seed Djokovic, of Serbia, survived a stern test from Croatian Cilic to post a 6-7, 7-5, 6-4, 7-6 victory.

Despite playing in his first major without the world's top ranking since the 2004 Australian Open, Federer has appeared fully focused on adding to his haul of 12 Grand Slam titles, extending his winning streak at Flushing Meadows to 30 consecutive matches with victory over Stepanek.

The Swiss star improved his career record against Stepanek to 5-2, avenging a loss in the previous meeting in the quarter-finals at Rome in May. "It's nice to beat him, but I think he did well," Federer said.

That was one of 12 losses this season for Federer, who has failed to claim either a Grand Slam or ATP Masters title en route to losing his top ranking to main rival Rafael Nadal. Once again, Federer displayed a strong return game, breaking Stepanek six times while raising his total in the tournament to 17. "I think I returned well, especially [on] the second serve," the Swiss star said. "I didn't allow him to really have that many different options, off his second serve, and he decided to stay back.

"I was good off the baseline. I think I moved well today and really hit some great shots when I needed them."

Next up for Federer will be Russian Igor Andreev, a 6-2, 6-4, 6-4 winner over 13th seed Fernando Verdasco of Spain.

A day after celebrating his 26th birthday with a late-night win over Ernests Gulbis, Roddick jumped all over Seppi with his booming serve, finishing with 19 aces while also breaking the Italian four times. A beaten finalist at Flushing Meadows in 2006, the eighth-seeded American has reached the quarter-finals in all but one of his seven previous appearances at the US Open.

Djokovic advanced after a gruelling encounter with the big-serving Cilic, who served 19 aces in a contest that last three hours and 52 minutes. The Croatian hit 47 winners in total but was ultimately undone by 12 double faults and 65 unforced errors.

Djokovic will face 15th-seeded Spaniard Tommy Robredo, who recorded a 7-6, 6-2, 6-3 win over 19th seed Jo-Wilfried Tsonga of France.