World number two Maria Sharapova withdrew from the WTA event in Montreal on Wednesday, citing a viral infection.
The Russian Olympic silver medalist joins US gold medal winner Serena Williams in missing the event, which falls directly after the tennis competition at the London Games.
"I'm sorry not to be able to play," Sharapova said. "I have a bad viral infection which I must have picked up after London."
Sharapova will be replaced in the draw by Galina Voskoboeva of Kazakhstan.
In second round matches Wednesday, fifth-seeded Czech Petra Kvitova rallied to beat Kazakhstan's Ksenia Pervak 4-6, 7-6 (7/0), 6-0.
Italy's Sara Errani, the eighth seed, cruised past Slovakia's Jana Cepelova 6-3, 6-2 and fellow Italian Flavia Pennetta advanced when South African Chanelle Scheepers retired with an injury.