Sam Querrey broke top seeded Andy Roddick's serve twice in the deciding set to stun the defending champion 7-5, 3-6, 6-1 in the quarter-finals of the Memphis Championships on Friday.
American world number 31 Querrey took charge of the deciding set after he battled to deuce in the second game before Roddick double faulted to gift his compatriot a 2-0 lead.
"Today I played one sloppy game in the third set and that was it," Roddick told reporters.
"Sometimes at this level there's not much in between it," he added.
Querrey said he knew he had to take advantage of Roddick's errors.
"You're not going to get that too many times from Andy, but I'll take it," he told reporters.
"When that rare opportunity comes you've just got to take advantage of it. I thought I did a good job there, and just kept the momentum going and played an unbelievable third set," he added.
Querrey consolidated his advantage with another break two games later and eased to victory for the loss of just one more game.
In the opening set, the big serving pair went toe-to-toe until the 11th game when Querrey broke Roddick to forge a 6-5 lead and took the opener in style with his 10th ace.
Roddick built on an early break in the second set to even the contest before Querrey, who sent down 16 aces overall, regrouped in the third to gain revenge for a semi-final defeat to the world number seven in San Jose a week ago.
"It was disappointing. I felt like it was the opposite of Sam's and my match of last week. I felt like I had the better end of the first two sets and (yet) we were even," said Roddick.