Wimbledon champion Marion Bartoli retired because of an abdominal injury early in the second set of her third-round Rogers Cup match against Slovakia's Magdalena Rybarikova on Thursday
Rybarikova led 7-6 (5), 1-0 when the seventh-seeded Bartoli stopped playing.
Playing her first event since returning from a hamstring injury that sidelined her since her major victory in London, Bartoli was in obvious discomfort as she walked to the player bench area, where she had a trainer stretch her arm and leg. On Wednesday, she beat American qualifier Lauren Davis 6-0, 6-3.
Earlier, fourth-seeded Li Na of China fought back from a three-game deficit in the final set for a 3-6, 6-1, 7-6 (5) victory over Serbia's Ana Ivanovic.
Third-seeded Agnieszka Radwanska of Poland also advanced to the quarterfinals, beating American Sloane Stephens 6-1, 7-6 (2). Fifth-seeded Sara Errani of Italy beat Alize Cornet of France 7-5, 7-6 (3).
Top-seeded Serena Williams faced Kirsten Flipkens of Belgium.