Ailing Stan Wawrinka won't play Rogers Cup

World No. 4 Stan Wawrinka is pulling out of the next week's Rogers Cup in Montreal due to a knee injury that he says has been bothering him all season.

Wawrinka, a semifinalist at last year's event, said on his Facebook page that he is missing the Rogers Cup and a following Masters 1000 event in Cincinnati until his knee is 100 per cent, possibly in time for the U.S. Open, where he is the defending champion.

He attempted to play with the sore knee at Wimbledon last month only to lose in the first round.

As a result of Wawrinka's withdrawal, Canadian tennis star Milos Raonic will now be seeded eighth and receive a first-round bye. Raonic will be seeking his first Masters 1000 title.

"After much backwards and forwards and consultations with my doctors and my team unfortunately I have decided to skip Canada and Cincinnati to be on the safe side even though I've been battling hard to make these events," Wawrinka posted on Facebook.

"Hugely disappointing for my fans and myself that I have to make this decision, but I need to be 100 per cent confident before I resume competition till the injury that plagued me in Wimbledon has been resolved. My team and I are doing everything possible to make this a speedy recovery."

"Stan has always been loyal to our tournament and we're obviously disappointed he won't be here this year," Eugene Lapierre, tournament director of the Rogers Cup in Montreal, said in a statement. "Stan is one of the best players in the world and we wish him a quick recovery and best of luck for the rest of the season."