Taylor Fritz moved into the semifinals of the Basel Open on Friday after Arthur Fils retired injured from their match.
The American was leading 6-2, 2-0 when the French world No 8 called it quits due to a potential back issue.
At the end of the first set, he called for the doctor – due to discomfort in his lower back according to television footage – and was forced to leave the court for a few minutes to receive treatment.
The flare-up comes at a bad time for the 22-year-old as he sits seventh in the race for the ATP Finals and is set to play his home Masters tournament in Paris next week.
Fritz, an outsider to reach the season-ending Finals next month at 12th in the race, will meet Monaco's Valentin Vacherot for a spot in Sunday's final in Switzerland.
Vacherot put an end to Holger Rune's run with a quarterfinal victory in three sets over the Dane, who is still making his way back from a lengthy injury lay-off.
Jiri Lehecka will play Novak Djokovic's vanquisher Jan-Lennard Struff in the other semifinal.
The world number 21 pulled off a 6-4, 6-2 win to end local wildcard Jerome Kym's fairytale campaign, while Struff beat Nuno Borges 6-2-6-3.