“I have been practising for a while now without serving and started with serves four days ago. Everything has been going well. However, yesterday, after my normal practice, I felt a slight bother on my abdominal and today it was still there,” Nadal said.
“After speaking with my doctor we prefer to take things in a conservative way and give a few more days before starting to compete.
“I am very sad not to finally travel to Montreal and to play a tournament that I have won five times and I love to play. I seriously hope I can come back there to play in front of the amazing crowds up there.”
Wimbledon champion Novak Djokovic also withdrew from the event on Thursday as he cannot enter the country without being fully vaccinated against Covid-19, while Britain's Andy Murray was handed one of four wild cards.