Tsitsipas, who was contesting the Masters 1000 final on his 20th birthday after four consecutive wins over top-10 players in Toronto, threw all he had at Nadal but was ultimately unable to beat the world number one.
"If you told me this two weeks ago I would not have believed it," Nadal said in his on-court interview. "It's a great way to start the hardcourt season. Winning in Toronto is so important. You don't win Masters 1000s very often. It's a very important victory for me and I'm very happy."
Tsitsipas did well to serve out at love in the first game but Nadal took over as he broke back in each of his next two chances before sealing the opening set in 34 minutes when the world number 27 sent a routine forehand into the net.
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The second set looked to be going much the same way until Tsitsipas mounted a late charge that briefly shifted momentum in his favour.
Nadal, serving for the match at 5-4 and having dropped only three points on serve the entire match up to that point, suddenly looked tight as Tsitsipas broke him and then held serve to pull within one game of levelling the contest.
"I got nervous, I played a terrible game," said Nadal after winning his first hardcourt Masters title since Cincinnati in 2013. "But that's how it is. You keep going. That's part of the sport.
"Everybody gets tight at some moments. Today for me was an important match and I just made a couple mistakes but the positive news is I played my best match without a doubt."
Nadal, much to the dismay of the many flag-waving Greek supporters in attendance, forced a tiebreak where he rallied back from a mini break and clinched the win with a crosscourt forehand winner for a record-extending 33rd Masters title.
The win made the 32-year-old Spaniard, who also won the Rogers Cup in 2005, 2008 and 2013, the fourth player in the professional era to win 80 titles, joining a club that includes Jimmy Connors (109), Roger Federer (98) and Ivan Lendl (94).
(Reporting by Frank Pingue; Editing by Clare Fallon and Pritha Sarkar)
