Andy Murray's high after winning Olympic gold was ended after the world number four withdrew from the Rogers Cup with a knee injury.
The second seed, who won the singles title at London 2012 with a straight-sets defeat of world number one Roger Federer, had been due to play Canadian Milos Raonic for a place in the quarter-finals.
Murray, who also picked up a silver medal in London after teaming up with Laura Robson in the mixed doubles, had felt soreness in his knee after his win over Italian Flavio Cipolla on Wednesday. "It's unfortunate. I think it would have been a fun match," said Murray.
The British number one added on www.atpworldtour.com: "I hope he has a good tournament. It will be great for him to go deep in this the tournament here for the first time."
After physio work Murray decided it was best not to risk further injury, with the start of the US Open less than three weeks away.
The Scot explained: "It was a bit sore yesterday, then I had some treatment after the match. It felt a bit better, and then came in today, I saw the physios, they did some work on it, and it's still a bit sore.
"I did a little warm-up on the bike to try it out and it was still sore, so I had to make a decision as to what I was going to do... I don't want to go full out and possibly do any more damage. That was why I decided not to play."
Raonic was able to pass on his best wishes to Murray when they spoke after the Canadian learned of his opponent's withdrawal.
He said: "[I] just wished him that he gets healthy quick, because he's been playing really well lately. It's my first quarter-final in a Masters , and to be at home is pretty special. I guess there is a give and take with it."