Top-seeded Gilles Simon of France withdrew from the Open de Nice on Wednesday, casting doubts over whether he will play at the French Open.
Simon, who retired from his second-round match in Rome this month, said the pain in his upper back was "too strong."
The French Open starts on Sunday.
The 13th-ranked Simon was set to face James Duckworth of Australia for a quarterfinal spot and was replaced by French lucky loser Quentin Halys, who lost 7-6 (0), 7-6 (5).
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Duckworth will be up against Borna Coric in the quarterfinals after the 53rd-ranked Croatian saved all three break points to beat qualifier Gianni Mina 7-6 (4), 6-3.
Fourth-seeded Leonardo Mayer advanced to the last-eight with a 6-4, 6-0 win over French wild-card entry Lucas Pouille to set up a meeting against Juan Monaco.
Dominic Thiem also progressed when sixth-seeded Nick Kyrgios retired with an elbow injury while trailing 4-3 in the first set.
"I expected a very tough match and I'm really sorry for Nick," Thiem said. "But I have to take it like this and of course I'm happy to be in the quarterfinals."
Next up for Thiem will be defending champion Ernests Gulbis of Latvia, who won his second match of the year in a 7-5, 4-6, 6-1 defeat of Alexandr Dolgopolov.
"There are still a lot of things that I need to work on," Gulbis said. "My best memories of my game were from the beginning of last year until the end of Roland Garros. Since then, I didn't have really good results and I didn't really feel well on the court because of different reasons, because of shoulder problems, because of some other reasons. Slowly it's going to change. I need to win these kinds of matches."
Also advancing was Serbia's Dusan Lajovic, who beat Pablo Carreno Busta 6-3, 4-6, 7-5 and will next take on second-seeded John Isner.