Federer's tough slog

in the fourth set at 5-6 and failed to hold with Robredo breaking to take the match into a decider.

But the world number three responded well to that setback, breaking Robredo in the fifth and ninth games to finally wrap up the win.

Djokovic next faces a quarter-final clash with eighth seed Andy Roddick of United States.

The American kept his hopes of a home major alive with a comprehensive 6-2 6-4 6-1 rout of 11th-seeded Chilean Fernando Gonzalez.

Meanwhile, Nikolay Davydenko succumbed to Muller despite making 23 less unforced errors for the match.

But an impressive 20 aces helped to ensure the 25-year-old from Luxembourg's progression to the quarter-finals.

Round 4, Men's Singles, US Open
2-Roger Federer (Switzerland) def 23-Igor Andreev (Russia) 6-7 (7-5) 7-6 (7-5) 6-3 3-6 6-3
3-Novak Djokovic (Serbia) def 15-Tommy Robredo (Spain) 4-6 6-2 6-3 5-7 6-3
Gilles Muller (Luxembourg) def 5-Nikolay Davydenko (Russia) 6-4 4-6 6-3 7-6 (12-10)
8-Andy Roddick (USA) def 11-Fernando Gonzalez (Chile) 6-2 6-4 6-1