He proved unable to take the match into a decider, as the 20-year-old's impressive tournament came to an end when Nadal sealed a 6-2, 7-6 win to take the title at the Association of Tennis Professionals Masters 1000 event. Nadal said afterwards. "I got a little bit tight and nervous". Winning in Toronto is so important.
Tsitsipas is attempting to become the first player since Albert Portas at Hamburg in 2001 to win his first ATP World Tour title at a Masters 1000 event.
Pouille outclassed a rusty-looking Murray in the opening set, breaking three times as the Briton sent down six double faults, recorded a first-serve percentage of 46, and won two out of 14 points on his second serve.
The second set looked to be going much the same way until Tsitsipas mounted a late charge that briefly shifted momentum in his favour.
Murray pulled it back to 5-4, having saved match point on his serve as he trailed 5-3, but Pouille held his nerve to hold serve in the 10th game, taking the set 6-4 and sealing a deserved victory.
"I am very sorry to announce that I won't be playing in Cincinnati this year", Nadal said on Twitter.
"But that's how it is".
"Hopefully with another couple of weeks and training, and I'm still, you know, quite early in this process, I think that's the seventh match I have played, so, you know, hopefully each week feel better and I can keep working on things".
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"Everybody gets tight at some moments. I'm excited about it and it's an opportunity for me to play a good match against one of the best players".
It was his 33rd Masters victory, meaning that, at 32 years of age, he now has more of these titles than years on the planet.
Nadal won the Western & Southern Open in 2013.
Top-ranked Simona Halep is fresh off another title.
"I remember I was watching from the television in Portoroz in the Challenger that I was playing that week, and I was watching his matches every single day".
"It's a pity Rafa is not playing, but it's still an incredible draw".
Tsitsipas broke and forced a second-set tiebreaker, where Nadal steadied the ship and elevated his game once again. But the American broke Halep with a forehand victor and held two set points on the Romanian's serve at 6-5.