Olympic silver medalist Maria Sharapova pulled out of the $2.168 million Rogers Cup tennis event in Montreal, citing a stomach ailment.
Sharapova, who was a Rogers Cup runner-up in 2009, told organizers she picked up a bug last week at the London Games.
The world No. 2 Russian star was the Olympic runner-up to American great Serena Williams four days ago and captured the French Open title in June to complete a career Grand Slam.
Sharapova was replaced in the Montreal draw by lucky-loser Galina Voskoboeva from Kazakhstan.
Australian Samantha Stosur had no problems advancing out of the second round as Romanian Simona Halep retired in the second set with a left thigh injury after Stosur won the first, 7-5. It was tied 4-4 in the second when Halep retired.
The U.S. Open champion Stosur was last year's Rogers Cup runner-up to Serena Williams in Toronto, as this tournament shifts between Toronto and Montreal each year.
Fifth-seeded Czech Petra Kvitova downed unseeded Ksenia Pervak of Kazakhstan, 4-6, 7-6 (7-0), 6-0.
In other second-round play, eighth-seeded French Open runner-up Sara Errani of Italy whipped Slovak Jana Cepelova 6-3, 6-2, South African Chanelle Scheepers moved on when 14th-seeded Italian Flavia Pennetta retired in the first set. Pennetta was up 4-1, but called it quits due to a right wrist injury.
Lucie Safarova, the 16th seed from the Czech Republic, dismissed Kazakhstan qualifier Sesil Karatantcheva 6-4, 6-4.
American Christina McHale overcame Dutch qualifier Arantxa Rus 3-6, 6-1, 6-3, while American Varvara Lepchenko leveled Russian Anastasia Pavlyuchenkova 6-2, 6-3 in round one.
In opening-round action Wednesday on the hardcourts at Uniprix Stadium, Dutch qualifier Kiki Bertens came back top best Russian Nadia Petrova 3-6, 7-6 (7-4), 6-4; Chinese Peng Shuai downed French qualifier Aravane Rezai 7-6 (7-5), 6-3; German Mona Barthel held off Hungarian qualifier Timea Babos, 6-4, 5-7, 7-6 (7-5); and Canadian wild card Eugenie Bouchard defeated Israeli Shahar Peer 3-6, 6-2, 7-5.
The 2012 Rogers Cup champion will pocket $385,000.