Roger Federer pulled out of the Rogers Cup on Friday, following three straight surprising exits from tournaments.
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The Swiss star gave no reason for the withdrawal from the tournament, which begins Monday.
"I am disappointed not to be playing in Montreal next week," Federer said in a statement. "It is a great tournament with amazing fans. I look forward to competing there in the future."
He won the Rogers Cup in Toronto in 2004 and 2006. Federer lost in the final to Novak Djokovic in Montreal in 2007.
Federer lost to No. 55 Daniel Brands last week in his opening match at the Swiss Open. In Hamburg, Germany, he cited back pain in his semifinal defeat to No. 114 Federico Delbonis. He fell to No. 116 Sergiy Stakhovsky in the second round at Wimbledon.
The winner of 17 Grand Slam singles titles dropped to fifth in the rankings after Wimbledon, marking the first time he has fallen out of the top four in a decade.
Ivan Dodig of Croatia and Pablo Andujar of Spain will enter the main draw in Montreal.
Viktor Troicki and Marin Cilic will not compete, joining Jo-Wilfried Tsonga and Mardy Fish on the sideline.