Four-times champion Roger Federer [Images] weathered a tough battle against Luxembourg qualifier Gilles Muller to book his place in the US Open [Images] semi-final with a 7-6, 6-4, 7-6 win on Thursday.

The second-seeded Swiss was tested to the full on a sweltering, breezy afternoon in New York by a player ranked 130th in the world before sealing victory in two hours 26 minutes.

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Seeking his fifth consecutive US Open title but his first Grand Slam crown of the year, Federer edged marathon man Muller 7-5 in the third-set tiebreak to win his 32nd consecutive match at Flushing Meadows.

"I'm happy I won, first of all," a relieved Federer told reporters after reaching a record 18th consecutive Grand Slam semi-final. "After a tough match I had last time, it's nice to get through (in) straight (sets).

"This was a tough opponent who has been playing very well in this tournament and he showed why he was so dangerous today.

"He served great, takes big chances from the baseline and there wasn't a whole lot of rhythm out there," added the Swiss, who scraped past Russian Igor Andreev in five sets in the fourth round.

"It made it more difficult with the wind and the sun. I'm very happy with this result."

Federer's opponent in the last four will be either American favourite Andy Roddick [Images] or third-seeded Serb Novak Djokovic [Images], who were scheduled to play their quarter-final on Thursday evening.

The opening set went with serve until the 12th game when Federer piled on the pressure and appeared likely to break his big-serving opponent. However Muller, trailing 5-6, did well to save five set points before holding for 6-all.

PULLED AHEAD

Federer, deposed as world number one two weeks ago by Spaniard Rafael Nadal [Images], raced into a 4-1 lead in the tiebreak but was pegged back to 4-4 before again pulling ahead.

The crowd gave both players a standing ovation after a breathtaking rally of aggressive shot-making finally ended with Federer winning the point to go 6-4 up. The Swiss clinched the first set at the seventh attempt.

Federer broke Muller in the ninth game of the second, working his opponent around the court before ripping a passing shot down the line to lead 5-4. He comfortably held serve to wrap up the second set.

The third set was also tight, each game going with serve to force another tiebreak which Federer won 7-5 when left-hander Muller netted a backhand.

Muller, who twice had to fight back from two sets down to book his place in a grand slam quarter-final for the first time, had bitter-sweet feelings over his defeat.

"The most amazing thing is that this is the first time in my career I'm actually winning a lot of matches," said the 25-year-old, who had been on court for four more hours than Federer before their contest.

"Before I used to win one big match and lost the next one because I lost focus and I was getting all fired up in my head because I won that (one) match.

"But now I'm just staying calm," added Muller, who upset then-world number three Roddick in the first round of the 2005 U.S. Open.

"Even today I guess a lot of people think I should be happy but actually I'm disappointed because today I had my chances, and I didn't take them."

Muller unleashed 16 aces and struck 46 winners, four more than Federer, but also piled up 32 unforced errors on the Arthur Ashe Stadium court.

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