Rafael Nadal's US Open preparations have been thrown into doubt after the world No 4 withdrew from the National Bank Open in Toronto with a lingering left foot injury.
The second seed was scheduled to play South African Lloyd Harris in the second round on Wednesday, but has been replaced in the draw by Spanish compatriot Feliciano Lopez.
The injury already forced the 20-time Grand Slam champion to miss Wimbledon and the Tokyo Olympics, and threatens to derail his US Open campaign.
In announcing his withdrawal, Nadal said the pain of the injury was not allowing him to enjoy being out on the court.
He said: "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. I need to go back and try to find a way to be better again.
"At the end of the day, for me the most important thing is to enjoy playing tennis. Today, with this pain, I am not able to enjoy it, and I really don't believe that I have the chance to fight for the things that I really need to fight for."
Nadal was able to participate in last week's Citi Open, but suffered a shock defeat to Harris in the third round.
Nadal added: "I really wanted to play here a lot, but now is the moment to make a decision.
"This is unfortunately the decision that I have taken, and probably in the next couple of days we are going to know more."
The US Open begins in New York on August 30.
