Roger Federer has withdrawn from next week's Masters 1000 event in Montreal as he struggles with a back injury.
The Swiss, world number five, has lost early in two of his last three tournaments, saying last week: "My main priority now is to fix my back."
Andy Murray will return to action for the first time since winning Wimbledon, while Novak Djokovic and Rafael Nadal are also in the line-up in Montreal.
"I am disappointed not to be playing in Montreal next week," said Federer, 31.
"It is a great tournament with amazing fans. I look forward to competing there in the future."
With just one major title to his name in the past three years, Federer had been trying out a new, larger racquet following his shock defeat by Sergiy Stakhovsky at the All England Club.
However, he revealed last week that his back has become a significant issue.
"I've had serious problems with the back," he said after his latest defeat. "I had to get some anti-inflammatories last week in Hamburg due to the pain. I will have to do a lot of exercises and see how it all feels."
Murray suffered his own back issues earlier in the year and was forced to miss the French Open, but since returning has not lost a match and picked up titles at Queen's Club and Wimbledon.
The 26-year-old took a five-day holiday in the Bahamas after his historic victory in SW19 last month, but then returned to his base in Florida to resume training as he prepares to defend a Grand Slam title for the first time at the US Open later this month.
The Scot is seeded second at the Rogers Cup in Montreal behind defending champion Djokovic, who is going for a hat-trick of titles in Canada.
Nadal was another to suffer and has admitted hard courts are punishing on his vulnerable knees, but the 27-year-old Spaniard won his only tournament on the surface this year in Indian Wells.
The top eight seeds in Montreal have byes in the first round, and Murray will begin on Tuesday or Wednesday against Grigor Dimitrov or Marcel Granollers.
He has been drawn in the opposite half to Djokovic and Nadal, and could face Juan Martin del Potro in the last eight and David Ferrer in the semi-finals.