Germany's Tommy Haas beat America's top seed Andy Roddick 6-3 6-4 to win the US Clay Court Championship in Houston.
Haas, who missed all of last year following shoulder surgery, was playing in his first final for two years. He last won an ATP event in 2001.
He broke the US Open champion's 12-match winning streak and extended his record to 4-0 against the American.
Roddick won the title in 2001 and 2002 and lost in the final last year to countryman Andre Agassi.
The unseeded Haas was shocked at his achievement.
He said: "It means a lot to have my first clay court title but even more because of what I've been through.
"To be honest, I didn't expect to win a tournament this quick but all you can do is prepare for each tournament and do your best and see what's possible.
"Overall, the shoulder feels pretty good," Haas said. "A match like this and a win like this over Andy does a lot for your confidence."
Roddick admitted that he had been second best on the day.
"Haas is a good player with a great feel for the game. He plays well with pace and he was playing well today.
"He just played better, it's as simple as that," said Roddick.