Former champion Andy Roddick [Images] suffered the humiliation of a first-round defeat at the U.S. Open on Tuesday, going down 7-6, 7-6, 7-6 to an inspired Gilles Muller of Luxembourg.

Starting brightly on his 23rd birthday, the American fourth seed served for the first set at 5-3 but Muller broke to love and then carved out three successful tiebreaks to clinch a famous win.

"I have no idea (how I did it)," Muller said of the victory on his U.S. Open debut.

"For me it was just unbelievable to come out here today. I told myself to just enjoy it and I did it every minute."

The world number 68 beat Spaniard Rafael Nadal [Images] at Wimbledon [Images] in June and even though he reached the final in Los Angeles last month, the 22-year-old was not expected to trouble Roddick on home soil.

Champion at Flushing Meadows in 2003, Roddick warmed up for the U.S. Open by winning the title in Washington and reaching the final of the Masters Series in Cincinnati.

However, in front of a packed crowd on Arthur Ashe Court, Muller outplayed Roddick in every department of the game, hammering 24 aces to the American's 17 and smashing 65 winners.

Roddick began brightly, breaking on his way to a 5-2 lead but after losing his serve to love, he was always struggling and Muller took advantage to wrap up the first set 7-4 in the tiebreak.

The American saved three break points in the fifth game of the second set but Muller had enough in reserve to eke out another tiebreak, 10-8.

A demoralised Roddick offered little resistance in the third-set decider, Muller sealing victory by a lop-sided 7-1 margin.

In the second round, Muller will face his doubles partner at this tournament, American Robby Ginepri.

"I played him five weeks ago, I think, in LA and I beat him," Muller said.

"I know it's going to be very tough because he's playing at home here and he's going to be very motivated, but I'll enjoy it."

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