Russian Maria Sharapova has pulled out of the this week's WTA Rogers Cup because of a strained chest muscle.
It comes a day after she withdrew from her JPMorgan Chase Open quarter-final in Los Angeles because of the injury.
The top seed received treatment but was advised not to hit any balls until Wednesday or Thursday.
"I'm glad that they didn't wait another day and continue to think about it and provide hope that it might have worked out," said event chief Stacey Allaster.
This is the latest blow to the tournament, which has already lost Wimbledon champion Venus Williams, Australian Alicia Molik, Russian Elena Bovina.
The event, which begins on Monday, still contains six of the world's top 10 players, including defending champion Amelie Mauresmo, Serena Williams and Belgian stars Justine Henin-Hardenne and Kim Clijsters.