Svetlana Kuznetsova suffered another blow to her hopes of defending the US Open title with a defeat to Gisela Dulko in the Rogers Cup in Toronto.
Kuznetsova, who has not won a title all year, required on-court treatment for a back problem during the 7-6 (7-3) 7-6 (10-8) defeat.
"It's hurting very much," said Kuznetsova. "I should have stopped."
A pain in her left knee forced Serena Williams to pull out of her match against Italy's Flavia Pennetta.
Defending champion and second Amelie Mauresmo went through safely.
Dulko v Myskina
Penetta v Clijsters
Vaidisova v Henin-Hardenne
Petrova v Mauresmo
The Frenchwoman beat Spain's Conchita Martinez 6-0 6-2 to set up a quarter-final meeting with Nadia Petrova, who was leading 6-2 3-0 when China's Li Na retired with a sprained right ankle.
Kim Clijsters also benefitted from a retirement when Serbian Ana Ivanovic pulled out of their third-round match with an injured right pectoral muscle.
Anastasia Myskina edged past Japan's Shinobu Asagoe 7-6 (7-5) 7-5 and will play Dulko in the last eight.
In the night match, fourth seed Justine Henin-Hardenne beat China's Jie Zheng 6-2 6-4.