American Andy Roddick suffered a third-round loss at the $550,000 ATP Tour's Winston-Salem Open, losing in straight sets to Belgium's Steve Darcis.
Darcis, who made it to the third round of the London Olympics, outlasted fifth-seeded Roddick in a pair of tiebreak sets, 7-6 (8/6), 7-6 (7/3) at the Wake Forest University facility.
The 28-year-old Darcis, who is ranked 81st in the world, was the last player to make the cut for the main draw.
"I didn't expect this when I came here; I wasn't playing well," said Darcis. "I've played two good first matches and now I have played well, serving well and trying to be aggressive when I could. Of course, it is a good win before the US Open."
He next faces second-seeded Tomas Berdych of the Czech Republic, who advanced with a 6-3, 6-2 win over Finland's Jarkko Nieminen.
The hardcourt tournament serves as a warm-up event for next week's US Open.
Roddick, a former world No. 1 and ex-US Open champion, is struggling heading into the last Grand Slam of the year at Flushing Meadows.
"I served okay, but I didn't return too well," said Roddick. "I didn't hit the ball clean and he played better when he had to. I wanted to play better here."
He lost early at last week's Cincinnati Masters tournament, where he complained of back problems. He has now dropped three of his last four matches after winning five straight, including a title in Atlanta.
Fourth-seeded Alexandr Dolgopolov of Ukraine moved into the quarter-finals with a 6-3, 6-4 win over Argentina's David Nalbandian.
Seventh-seeded American Sam Querrey continued his run of good form with a 6-3, 6-4 victory over Spaniard Feliciano Lopez. Querrey will meet Dolgopolov in the quarter-finals on Thursday.