Nice - Top-seeded Gilles Simon of France withdrew from the Open de Nice on Wednesday, casting doubts on his French Open participation.
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 will be replaced by French lucky loser Quentin Halys.
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 quarter-finals."
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.
Also advancing was Croatia's Borna Coric, who beat qualifier Gianni Mina 7-6 (4), 6-3.
Results from the ATP Nice Open on Wednesday (x denotes seeded player):
James Duckworth (AUS) bt Quentin Halys (FRA) 7-6 (7/0), 7-6 (7/5)
Borna Coric (CRO) bt Gianni Mina (FRA) 7-6 (7/4), 6-3
Leonardo Mayer (ARG x4) bt Lucas Pouille (FRA) 6-4, 6-0
Dominic Thiem (AUT) bt Nick Kyrgios (AUS x6) 4-3 - retired
Ernests Gulbis (LAT x3) bt Aleksandr Dolgopolov (UKR) 7-5, 4-6, 6-1
Dusan Lajovic (SRB) bt Pablo Carreo-Busta (ESP) 6-3, 4-6, 7-5