Sharapova also struggled with a shoulder injury at the French Open
Maria Sharapova has withdrawn from the WTA Rogers Open in Montreal with a shoulder injury which could jeopardise her Olympics and US Open participation.
The world number three will have an MRI scan on Thursday after struggling with pain in her second-round 7-5 5-7 6-2 win over Poland's Marta Domachowska.
Sharapova, 21, said her presence at the Olympics and then US Open in August depends largely on the test results.
Elsewhere, top seed Ana Ivanovic stuttered to a win over Petra Kvitova.
"It's obviously unfortunate," said Sharapova after confirming her withdrawal on Wednesday.
"You try to do every little possible thing you can. But at the end of the day when you go on court and you're thinking about aggravating things, you're thinking about an injury, it's not really the way to play.
I'm too good of a player to go out there and try to fight through something that can eventually become something serious
"It's tough to go on court and not be close to even 50%.
"I'm too good of a player to go out there and try to fight through something that can eventually become something serious."
Playing with a swollen thumb, world number one Ivanovic advanced to the third round with a 6-3 4-6 6-3 win over Czech 18-year-old Kvitova.
The top seed and 2006 champion, who had a first-round bye, overcame nine double faults and the loss of her serve five times against a player who beat Venus Williams in Memphis earlier this year.
Serb compatriot and world number two Jelena Jankovic cruised through with a 6-0 6-4 win over Aleksandra Wozniak of Canada.
Ivanovic, making her first match since crashing out of Wimbledon in the third round, must finish higher than Jankovic and Sharapova in Montreal to cling on to the world number one ranking she has held since winning the French Open in early June.
Sharapova needed a trainer to look at her right shoulder in the middle of the second set against Domachowska in a match which lasted nearly three hours and was strewn with double faults, the Russian making 17 in total.
She has struggled this year with a shoulder injury that forced her, ahead of the French Open, to withdraw from the Italian Open in June - though she did not appear 100% comfortable in Paris.
Sharapova also withdrew from the Sony Ericsson Open in Miami in March, again citing a shoulder problem.
Ivanovic, who refused to use her swollen thumb as an excuse for her poor display, will play Tamira Paszek of Austria in the third round after she beat Melinda Czink of Hungary 6-4 7-5.
Sharapova was set to meet Japan's Ai Sugiyama while Jankovic will face another Canadian in the shape of Stephanie Dubois, who upset number 13 seed Maria Kirilenko of Russia 6-2 2-6 7-6 (7-4).
Also advancing from the second round were Russian seeds Anna Chakvetadze, Dinara Safina and Nadia Petrova, along with 15-year-old Michelle Larcher de Brito.