Top seed Juan Martin del Potro's comeback from injury ended in the quarter-finals of the Delray Beach Open following a 6-4 3-6 7-6(5) defeat by American Mackenzie McDonald.
Del Potro, playing his first tournament since he injured his knee at the Shanghai Masters in October last year, played with a heavily strapped knee on Friday. He also pulled out of next week's Mexican Open in Acapulco, a tournament he won last year.
"I wanted to do a big effort tonight. I fought until the last ball and had many opportunities, but it's hard to win matches when you can't move at 100 percent," Del Potro, whose career has been blighted with injuries, told reporters.
"The doctors say I need time if I want to be 100 percent, but I don't want to be home watching the tournaments on TV. I've done that before and it was really bad for me.
"I played three matches after four months away and that's the best thing for me. I know how good I can be when I'm ready, but I'm not in the physical condition to play at this level yet."
Del Potro fired 16 aces in the match but could not outlast the 23-year-old McDonald with his injured knee visibly slowing him down. The Argentine managed to defend two match points in the third set tiebreak before eventually bowing out.
It was McDonald's biggest win of his career and the first over a player ranked in the top five, as he moved into his first ever ATP Tour semi-final where he will take on Moldovan Radu Albot.
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