Sharapova told organizers she apparently caught a stomach bug in London during her silver medal performance in the Summer Olympics.
The Russian star is ranked No. 2 in the world. Her slot at the Rogers was filled by Galina Voskoboeva of Kazakhstan.
Stosur advanced to the third round when her Romanian opponent, Simona Halep, retired in the second set of their match Wednesday. The fourth-seeded Australian won the first set 7-5 before Halep injured her left thigh.
Also advancing in second-round play Wednesday were fifth-seeded Petra Kvitova and eighth-seeded Sara Errani of Italy. Christina McHale of the United States beat Dutch qualified Arantxa Rus 3-6, 6-1, 6-w in first-round action Wednesday.