Roger Federer was unable to mount a challenge in the second set
The Wimbledon champion's movement was severely restricted in the Rogers Cup final against Alexander Zverev.
The 36-year-old lost the first set 6-3 to his talented German opponent but midway through the second set lost power on his serve.
He suddenly appeared out of sorts allowing Zverev to break serve in the seventh game to pull ahead 4-3 and then hold on for a surprise win that took just 68 minutes.
Federer has struggled with back problems in the past and now faces a race against time to be fully fit before the US Open begins on August 28.
Wimbledon winners through the years
Sun, July 16, 2017
Express Sport takes a look back at all those players who have won at WImbledon over the years.
Play slideshow . Images 1 of 50
2017 – Garbine Muguruza
Eurosport pundit and former professional tennis player Annabel Croft reckons it was a recurrence of that back injury that left Federer stricken during the second set.
"There was a point which Zverev ended up winning where [Federer] was pulled from pillar to post and had to change directions quite a bit," Croft said
"I don’t know whether he tweaked his back but after that when he was serving, suddenly the motion went right off.
"There was no rhythm in it and he wasn’t able to rotate in the back and shoulders.
Roger Federer was left unable to move around the court properly by an apparent back injury
"The speed of the serve came right off and it was very, very apparent. On the returns of serve he just wasn’t moving.
"Even the walk around the court was a little bit reminiscent of how Andy Murray was walking at Wimbledon."
His obvious injury took the gloss of a fine win for Zverev who was poised and aggressive off the ground from the outset rarely giving Federer time to dictate.
The 20-year-old German arrived in Montreal fresh off his triumph in Washington and the latest win improved his record to 5-1 in finals this year with the only defeat coming at the hands of Federer at Halle.