Former World No 1 Lleyton Hewitt was knocked out of the US Men's Clay Court Championship in the first round.
Argentina's Martin Alund is ranked 12 places below Hewitt at 94 in the world but recovered from losing the first set to triumph 4-6 6-3 6-1 in Houston.
Hewitt broke Alund in the seventh game of the first set and then secured the set with an overhead winner.
The duo went on to trade breaks in the second and third games of the second set, before Alund took charge.
Hewitt was broken once again in the sixth game and Alund then saved a pair of break points before capturing the second set in the ninth game.
He went on to dominate the final set and, although Hewitt escaped three match points in the final game, he then served a double fault to bow out.
Hewitt had made a tiring trip over from Uzbekistan, having helping Australia to a 3-1 victory in a Davis Cup contest there.
"It's tough to get back from there to play the next week," he said. "I just didn't feel close to 100 per cent.
"It was a stomach thing and it's frustrating. If I could have gotten through this match, I could have felt better as the week goes on. It wasn't to be."
Dominic Inglot followed fellow Brit Jamie Murray into the doubles quarter-finals as he and his Filipino partner Treat Huey, seeded third, defeated Fernando Verdasco and Mischa Zverev 6-3 2-6 10-5.