McDonald, right, will play in his first ATP Tour semi-final
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 match points to win the second set, but McDonald converted his third match point in the 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 fourth-seeded American 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 American John Isner.
Del Potro moves on to next week's tournament in Acapulco, Mexico, where he accepted a wild card to defend his title before he plays at March's Indian Wells Masters, which he also won last year.
