Juan Martin del Potro's injury comeback ended with a three-set quarter-final defeat by American Mackenzie McDonald at the Delray Beach Open.
The Argentine top seed was beaten 6-4 3-6 7-6 (7-5) by the world number 84.
Del Potro was playing his first tournament since breaking a kneecap at the Shanghai Masters in October.
He defended two break points before winning the second set, but McDonald then converted his third match point in a third-set tie-break to seal victory.
McDonald, 23, next faces Moldova's Radu Albot, who came from a set and a break down to beat American fourth seed Steve Johnson 1-6 6-3 7-6 (7-5).
In the other last-four tie in Florida, Britain's Dan Evans will play his first ATP Tour semi-final for over two years against John Isner of the United States.
Del Potro appeared to be affected by a heavily strapped knee and he pulled out of next week's tournament in Acapulco, Mexico - where he had accepted a wildcard to defend his title - after the defeat by McDonald.
"It's hard to win matches when you can't move at 100%," he said.
The world number four is due to play at March's Indian Wells Masters, which he also won last year.