Third seed Robby Ginepri was the highest-ranked casualty in the first round of the US Men's Clay Court Championships in Houston on Monday, when the American lost 6-4, 6-4 to Spain's Albert Montanes.
Hampered by erratic play for a majority of the match, Ginepri's defeat was the latest setback in a poor start to a season that has garnered just two wins in 10 matches.
After dropping serve early in the second set, the American did manage to break Montanes in the sixth game but the Spaniard broke Ginepri again in the seventh game and maintained the advantage to close out the match.
In the day's other match involving a seeded player, fifth-ranked Tommy Haas of Germany eased into the second round with a 6-1, 6-4 victory over Spain's Oscar Hernandez.
The former champion dominated the first set and fought off a determined Hernandez in the second as he looks to pick up a second Houston title after his victory two years ago.
It was the first career meeting between the two players, with Haas improving his match record to 21-5 this season with the win.
The in-form German has already picked up titles at Delray Beach and Memphis this season.
In other matches, Olivier Marach of Austria defeated Juan Antonio Marin of Costa Rica 6-4 6-1 and Jurgen Melzer of Austria downed Ivo Minar of the Czech Republic 7-6, 6-0.
Top seed Andy Roddick of the US plays his first match of the $380,000 tournament against Paul Capdeville of Chile on Tuesday.