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Roger Federer is in serious doubt for the US Open after withdrawing from two lead-up events because of lingering injury concerns.
Federer's withdrawal from upcoming hard-court tournaments in Toronto and Cincinnati were announced on Thursday, with officials citing lingering issues with his surgically repaired knee.
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The news follows the 20-time grand slam champion's shock decision to skip the Tokyo Olympics after he said he "experienced a setback" with his knee at Wimbledon.
Federer, who turns 40 on Sunday, has not competed since losing in the quarter-finals to Hubert Hurkacz at the All England Club.
The Swiss champion was humbled in straight sets, losing the last set 6-0 to mark his first-ever 'bagel' at the grass-court grand slam.
Play begins in Toronto on Monday and at Cincinnati on August 16.
The hard-court events traditionally serve as the two primary tune-ups ahead of the US Open, the year's last grand slam that's scheduled to begin in New York on August 30.
Federer is a seven-time champion in Cincinnati and has won two titles in Canada.
But his withdrawal from both has cast serious doubts over his participation at the US Open, leaving fans shattered.
Roger Federer's horror run of injuries
Other players whose withdrawals from Toronto were announced on Thursday were Olympic gold medallist Alexander Zverev, Wimbledon runner-up Matteo Berrettini, Canadian Milos Raonic and Spain's Pablo Carreno Busta.
Novak Djokovic pulled out last month before his first match at the Olympics.
Arthur Ashe Stadium was the site of an epic run of five consecutive US Open titles for Federer from 2004 through 2008.
He reached the final again in 2009 only to lose in five sets to Juan Martin del Potro.
He has reached the US Open final only once since then, losing to Djokovic in the 2015 championship match.
Federer had two operations on his right knee in 2020 and went more than a full year between matches.
He returned to slam action at the French Open in late May and then pulled out after three victories, saying he wanted to be rested and ready for the grass circuit - especially Wimbledon.
When he later posted on social media that he would miss the Olympics, Federer wrote: "I have already begun rehabilitation in the hopes of returning to the tour later this summer."
Federer, currently ranked World No.9, is level with Djokovic and Rafa Nadal on 20 career grand slam titles, with Djokovic heavily favoured to make it 21 at the US Open.
The World No.1 Djokovic will also be trying to complete the first men's calendar-year grand slam since Rod Laver in 1969.
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