Second-seeded German Tommy Haas was upset 5-7, 6-4, 6-4 by Russian Igor Kunitsyn in the opening round of the US Clay Court Championships on Tuesday.
Haas, sidelined since withdrawing from the quarter-finals of last month's Pacific Life Open because of a sinus infection, lost a match lasting two hours 48 minutes.
The 26-year-old Kunitsyn, ranked 111th in the world, was the steadier player overall and Haas served up 12 double faults along with seven aces.
Kunitsyn will next meet Argentina's Sergio Roitman who beat big-serving American John Isner 3-6, 7-6, 6-3.
Third seed Mardy Fish, champion in 2006, eased into the second round with a 7-6, 6-3 win over Germany's Rainer Schuettler.
American Fish, who stunned world number one Roger Federer on his way to the Pacific Life Open final, broke his opponent twice to seal victory in 82 minutes.
In other matches, fifth-seeded Agustin Calleri of Argentina beat American Vince Spadea 6-4, 6-3 and seventh-seeded Spaniard Marcel Granollers crushed Latvia's Ernests Gulbis 6-2, 6-0.