Federer was solid if unspectacular in the face of some powerful hitting from the unfancied Luxemburger to close out the match 7-6 (7/5) 6-4 7-6 (7/5).

The four-time defending champion was expected to sweep aside Muller, who had enjoyed a dream run after being ranked 130 in the world before the tournament.

The contrast in the two players' careers was none better illustrated by the fact Federer was gunning for this 18th consecutive appearance in a Grand Slam semi-final, while Muller's best effort was a third-round showing at Wimbledon three years ago.

However, Muller coupled his heavy hitting with the same steely resolve that had helped him edge a five-set thriller against fifth seed Nikolay Davydenko in the previous round to cause the Swiss an uncomfortable afternoon in sweltering conditions on Arthur Ashe Court.

Federer will meet Novak Djokovic in the last four after the third seed overcame Andy Roddick 6-2 6-3 3-6 7-6 (7/5).

While other top-ranked players have struggled lately, Roddick had looked unstoppable since his second-round scare against Ernests Gulbis of Latvia.

But Djokovic raced into a two-set lead in the quarter-final clash before Roddick battled back to claim the third. A closely contested fourth went into a tiebreak and the 26-year-old American was just two points away from taking the match into fifth set at 5-4.

However, Roddick double-faulted twice in a row before Djokovic served to win the match.

"I'm really happy," said Djokovic. "To win against him (Roddick) is a huge effort."