Top seed and world number three Zverev, this year's French Open champion and Wimbledon runner-up, was toppled by Dutchman Tallon Griekspoor, who rallied for a 6-7 (3/7), 6-2, 6-4 second-round upset.

"I didn't expect to play well," Zverev said. "I also didn't expect to play this bad, to be honest."

Montreal native Auger-Aliassime had to withdraw before his scheduled night match after injuring his back during training on Monday.

"It was quite sudden. I was training here for two weeks and I had zero problems," Auger-Aliassime said. "Just the timing is tough.

"I thought that day by day things would get better, but I tried to go on court today, I tried to warm up, and then I basically couldn't serve really, so it was impossible for me to think about even competing."

Joining his fellow elites on the sidelines was fourth seed Daniil Medvedev, the 2021 champion, who went out 6-3, 7-6 (7/5) to Botic Van de Zandschulp, another Dutch spoiler.

Medvedev missed out on achieving his 100th career victory at the Masters level as he lost in his Canada opener, weighed down by 40 unforced errors.

Tournament officials already were fuming at an event in which world number one Jannik Sinner and Novak Djokovic were both high-profile absentees even before the start.

"I'm not going to lie, we were hoping that Felix was going to win at least a few matches," tournament director Valerie Tetreault said.

"He's definitely the player everybody was talking about and people wanted to know when he was going to play."

Zverev was competing for the first time since his Wimbledon final loss to Sinner and began to fade in the second set against world number 69 Griekspoor.

"This was probably the worst match of the season," Zverev said. "This one was up there as the worst.

"Tennis comes with ups and downs. I'm very happy with how the last couple of months were. Now it's about getting back to that form."

The Dutch winner came within a point of a 5-2 lead in the third set, but found himself square at 4-all as he lost serve.

But the underdog broke the seed straight back and served out the match in one of his biggest victories a game later.

"This is one of the most special wins I've ever had," he said. "I've had a tough season. I was close to throwing in the towel a few times. But I've been working harder in practice and reaching a good level.

"I felt it was about time for a big win."

Fifth-seeded defending champion Ben Shelton rallied from 4-1 down in his second set to overhaul US compatriot Jenson Brooksby 6-3, 7-5, winning six of the last seven games for his 28th victory of the season.

A huge positive

The son of a former ATP pro has won seven straight matches in Canada, taking 96 minutes to go through to the third round. He's 15-3 against countrymen since the start of 2025.

"Defending is tough. I had to get over pre-tournament nerves but I got my first straight-set win of the summer season," Shelton said.

"Jenson is a nightmare match-up for me. To win it is a huge positive."

Greek Stefanos Tsitsipas fell 7-6 (7/3), 7-5 to 19-year-old Brazilian Joao Fonseca in the second round.

Fonseca, who fired nine aces among his 31 winners, next plays ninth seed Casper Ruud.