Federer and Djokovic advance

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 (7/4) victory over Italian Andreas Seppi, and third seed Novak Djokovic of Serbia, who survived a stern test from Croatian Marin Cilic to post a 6-7 (7/9) 7-5 6-4 7-6 (7-0) 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.

"I think I returned well, especially (on) the second serve," Federer 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 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 lasted 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 (7/2) 6-2 6-3 win over 19th seed Jo-Wilfried Tsonga of France.

Seeking his third straight semi-final appearance at the US Open, fifth seed Nikolay Davydenko posted a 6-2 7-6 (8/3) 6-3 victory over Russian compatriot Dmitry Tursunov.

Davydenko has lost in the penultimate round at Flushing Meadows in the last two years to Federer and could meet the Swiss again in the quarter-finals although he must first get past surprise package Gilles Muller. Muller entered the US Open with just three wins this season but the player from Luxembourg advanced with a hard-fought 6-7 (3-7) 3-6 7-6 (7/5) 7-6 (8/6) 7-5 victory over 18th seed Nicolas Almagro of Spain.