Denis Shapovalov defeated Stefanos Tsitsipas 76 76 to clinch Canada's opening tie in the inaugural ATP Cup men's team event.
Earlier, Felix Auger-Aliassime claimed a breezy 6-1-6-3 win over Michail Pervolarakis.
Tsitsipas was considered the favorite going into the match against Shapovalov having defeated Roger Federer and Dominic Thiem to win the ATP Finals in November.
But Shapovalov was the better player on the big points against the Greek.
After more than two hours of power-hitting from two of the ATP's most exciting next generation players in a match in which there was no break of serves, Shapovalov converted his first matchpoint when Tsitsipas double faulted on his serve.
"It's definitely a huge win for me," Shapovalov said. "Obviously, he had an unbelievable end to the season and he's definitely one of the top players in the world right now. He's got a great game.
"So to beat a guy like this in the first match of the year, it's really special for me. It means a lot.
"I think today it was just a couple points here and there, just tried to keep playing my game, keep playing every single point and wait for my chances and I just think that I took advantage really well in the tie-breaks."
Info and photo from Reuters
