Serena Williams has withdrawn from next week's Rogers Cup in Montreal, citing shoulder inflammation.
The world number one, a three-time winner of the event, announced the news on Sunday - 24 hours before the tournament's opening day.
"Due to inflammation in my shoulder, I unfortunately must withdraw from the Rogers Cup," said Williams in a statement reported by the WTA Tour website.
"I was looking forward to competing in Montreal and I look forward to returning soon."
Tournament director Eugene Lapierre said the decision was understandable with the Olympic Games in Rio just two weeks away.
"Of course, we are disappointed that Serena will not play in the tournament this year. The fans really enjoyed the time she spent in the city in 2014," said Lapierre.
"Because this is an Olympic year, the players have very full schedules. Sometimes your body needs rest. We hope that Serena will recover quickly and wish her much success for the rest of the season."
Defending champion Belinda Bencic and 2010 winner Caroline Wozniacki are among other absentees in Montreal, while Andy Murray, Roger Federer and Rafael Nadal have all pulled out of the corresponding men's event in Toronto.