Japanese world number one Naomi Osaka retired from her Cincinnati Masters quarter-final with an apparent knee injury on Friday.
Osaka, who was playing American Sofia Kenin, sought treatment on the knee in the third set, then pulled out. Kenin was ahead 6-4 1-6 2-0 at the time.
The injury comes less than two weeks before the start of the US Open, where Osaka is the defending champion.
Osaka ironically returned to the top of the rankings last Monday after Kenin beat Ashleigh Barty in the second round of the Rogers Cup.
The defending US champion was forced to retire after a lengthy medical time out, and Kenin now awaits the winner between No.16 seed Madison Keys and Venus Williams.