Emma Raducanu suffered an injury scare just four days before the French Open in a three-set defeat to Danielle Collins on Wednesday.
The Briton, after winning the first set, was 5-0 down in the second when she called a medical timeout for treatment on a back problem.
While she finished the match, the 22-year-old won just four more games against Collins, with the American winning 4-6, 6-1, 6-3 to reach the quarterfinals.
Collins will play top seed Jessica Pegula or world No. 30 Anna Kalinskaya in the last eight.
Raducanu has been impressive on clay recently -- winning three matches at the Italian Open last week and dismantling Daria Kasatkina to start the Strasbourg Open -- but may now be sweating over her fitness ahead of the French Open.
The tournament begins on Sunday at Roland Garros, with the first-round draw on Thursday.