Delray Beach - American Mackenzie McDonald disrupted Juan Martin del Potro's latest injury comeback on Friday with a three-set victory over the top-seeded Argentinian in the ATP Delray Beach Open quarter-finals.
McDonald, ranked 84th in the world, outlasted the fourth-ranked Del Potro 6-4, 3-6, 7-6 (7/5) to notch his first victory over a top-five player and reach his first ATP Tour semi-final.
Del Potro, playing his first tournament since hobbling out of the Shanghai Masters with a broken right kneecap in October, took the court with his right knee more heavily bandaged than in his prior matches this week.
He was moving tentatively as he dropped the opening set, but still had plenty of pop on his serve and forehand as he managed to fend off two break points on the way to taking the second.
It would take a third-set tiebreaker to settle things, McDonald gaining the win with an overhead smash on his third match point.
McDonald, 23, next faces Moldova's Radu Albot, who rallied from a set and a break down to beat fourth-seeded American Steve Johnson 1-6, 6-3, 7-6 (7/5).
Del Potro was scheduled to continue his comeback bid at next week's tournament in Acapulco, but he said Friday night he would not be travelling to Mexico to try and defend his title there.
He is still hoping to defend his Indian Wells Masters title in March.
In other matches, US second seed John Isner blasted 24 aces in a 7-6 (7/2), 6-4 victory over eighth-seeded Adrian Mannarino.
Ninth-ranked Isner, who has dropped only one set in seven career wins over the French left-hander, needed only 93 minutes to book a spot in the last four against British qualifier Daniel Evans.
"I thought Adrian was playing well today. Those three break points I saved in the first set felt like set points," Isner said. "I was able to get the one break in the second set and sometimes that’s all you need."
Evans, ranked 1498th in the world defeated Italian sixth seed Andreas Seppi, ranked 52nd, 6-4, 6-4.
Evans, who beat defending champion Frances Tiafoe in the first round, is into his first ATP Tour semi-final since the 2017 Sydney International.
Results from Friday's matches at the ATP Tour's Delray Beach Open (x denotes seed):
Mackenzie McDonald (USA) bt Juan Martín Del Potro (ARG x1) 6-4, 3-6, 7-6 (7/5)
Radu Albot (MDA) bt Steve Johnson (USA x4) 1-6, 6-3, 7-6 (7/5)
Daniel Evans (GBR) bt Andreas Seppi (ITA x6) 6-4, 6-4
John Isner (USA x2) bt Adrian Mannarino (FRA x8) 7-6 (7/2), 6-4