Rafael Nadal has withdrawn from the National Bank Open with an ongoing foot injury.
The world number four had been absent for two months and pulled out of Wimbledon and the Olympic Games in order to recover from the injury picked up at the French Open in May.
He returned for the Citi Open in Washington last week as the hard court season gets in full swing ahead of the US Open, the final Grand Slam of the year.
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But after aggravating his injury in a second round win over Jack Sock, the Spaniard then lost in the third round to South Africa's Lloyd Harris in three sets.
The 35-year-old was due to play at the Masters tournament in Toronto, where he is a five-time champion, but has been forced to pull out of the competition which is already underway.
"I really wanted to play here a lot, but now is the moment to make a decision, and this is unfortunately the decision that I have taken," the 20-time Grand Slam champion said in a video message.
"I have had this issue for a couple of months, as people know. Of course, it is not a happy situation after all the success that I had here in Canada, not being able to play this year after missing a year.
"It's a tough one, but that's how it is today."
Roger Federer pulled out of the tournament as well as the Western & Southern Open with a knee injury.
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Novak Djokovic withdrew from the Cincinnati event in order to spend time with his family and recuperate before bidding to become only the sixth singles player in history to complete a calendar Grand Slam.
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