Andy Roddick crashed out of the US Open in the first round when he lost in straight sets on his 23rd birthday.
The former champion was beaten 7-6 7-6 7-6 by unheralded Gilles Muller at Flushing Meadows.
Roddick, seeded fourth, had no answer to the world number 68 from Luxembourg, who matched the 2003 champion shot for shot.
Meanwhile, Roger Federer had little trouble as he embarked on his defence of the US Open title with a comprehensive first-round victory against Ivo Minar.
The world number one, who beat Lleyton Hewitt in last year's final, progressed past his Czech opponent with a 6-1 6-1 6-1 victory.
Number 22 seed Mario Ancic was another straight-sets winner on day two, beating Hyung-Taik Lee, 6-2 6-1 7-5.
