Defending champion Alex de Minaur advanced to the final of the Mexican Open when Jack Draper retired in the third set.
The third-seeded Australian won the first set 6-3, dropped the second 6-2 and was leading 4-0 in the third when Draper, who was visibly diminished physically, called it quits.
De Minaur will face Casper Ruud, who pulled off an upset by beating second-seeded Holger Rune 3-6, 6-3, 6-4 in the late match.
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“I’m happy to be in the final, but I’m hoping that Jack is doing fine. He has a tremendous talent and hopefully he can recover for the upcoming tournaments,” de Minaur said.
“It’s hard to play when your opponent is not OK physically, but you need to remain focused.”
The 25-year-old de Minaur won his ninth straight match in Mexico and became the first defending champion to reach the Acapulco final since David Ferrer in 2013.
De Minaur has seven career ATP titles, with last year at Acapulco his most recent.
Draper, from England, was playing in his first semi-final of an ATP 500 tournament.
Later, in the other semi-finals, Ruud lost the first set and then was down 3-1 in the third with Rune serving, but he managed a couple of breaks and hold his serve to take the win.
“I did not get the best start, and the third set looked really bad, I just tried to stay in there and it seemed like Holger was struggling and at the end it worked for me,” Ruud said.
The 25-year-old Norwegian will try to win his 10th career title, the first one on a ATP 500.
Rune, who is the seventh player on the ATP rankings, lost in the Acapulco semifinals for the second year in a row.