Naomi Osaka, her US Open title defense due to start in just 11 days, retired with a left knee injury against Sofia Kenin in the quarter-finals of the WTA-ATP Cincinnati Masters.
The Japanese star, who regained the world number one ranking on Monday, had just forced a third set against Kenin and was serving at 0-1 in the third when she called for a trainer.
She was examined for nearly 10 minutes and after having the knee taped Osaka attempted to play on. But with her movement impaired she retired, Kenin advancing 6-4, 1-6, 2-0.
Kenin will face either Venus Williams or Madison Keys for a place in the final.