Roger Federer lost in the opening two rounds at the Halle Open for the first time in his career as his hopes of winning the title for the 11th time were ended by Felix Auger-Aliassime.
Federer beat Ilya Ivashka in his opening match of the grass-court season at a tournament defined by his dominance in Halle.
He came into Wednesday's clash with Auger-Aliassime boasting a 32-0 record in the first two rounds, and that looked set to be preserved after the 20-time grand slam champion won the first set.
But Auger-Aliassime, exactly 19 years Federer's junior, staged a stunning fightback in a contest with the largest age difference of the Swiss' 1,521 career matches.
His remarkable turnaround saw the Canadian claim a 4-6 6-3 6-2 triumph that marked the fourth top-10 win of his career.
It also meant he denied Federer a 70th match win in Halle and his 18th quarter-final in as many appearances at the tournament.
"It's a great victory, it's good for my confidence. It was already a great challenge for me to play a player like Roger, but to beat him, it's a great thing," Auger-Aliassime said. "It makes me really happy. But at the end, it's the quarter-finals in two days.
"If it was the final, then I'd be really happy… it's another step in the tournament, it's a great match, so hopefully I can keep on going like that.
"In the first set, I didn’t think I could have played much better outside of just missing one forehand in my service game and then he hit two amazing passing shots and I was just like 'Whoa'.
"I understand how good he is and how good he was when he was No. 1 in the world, so it was tough.
"Everything worked well today, to be honest. I think that's what you need to do against Roger. On my part, I served really well. I was able to put in a lot of returns, mix up coming to the net, closing well. I think overall I did a good match."
Marcos Giron is next for Auger-Aliassime after he overcame Jan-Lennard Struff in three sets. There were straight-sets wins for Andrey Rublev over Jordan Thompson and Philipp Kohlschreiber over Corentin Moutet. Rublev will face Kohlschreiber in the last eight.
Wednesday's play at the Queen's Club Championships saw Alex de Minaur win an all-Australian clash with John Millman, while Britons Cameron Norrie and Jack Draper defeated Aslan Karatsev and Alexander Bublik respectively.