Tsitsipas won the first two points of the game and converted his second break-point opportunity to get back on serve.
"I got nervous, I played a bad game", said Nadal after winning his first hardcourt Masters title since Cincinnati in 2013.
He improved to 4-0 in Rogers Cup final appearances.
"It feels like I'm playing Davis Cup and everyone is backing me up and they're so supportive", Tsitsipas said. "It means a lot".
Nadal regained control, earning a match point on a Tsitsipas forehand error.
"On the one match point that I had, he came up with an unbelievable backhand cross-court victor - it was a very impressive effort from him".
"No other reason than personally taking care of my body and trying to keep as healthy as I feel now", Nadal said in a statement.
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).
Sevilla threaten to pull out of Super Cup against Barcelona
The victor of the match that will be staged in Tangier, Morocco, will be decided over one leg for the first time . It follows a summer in which Dembele has regularly been mentioned as a potential departure from the Nou Camp.
They have been drawn against French singles players Richard Gasquet and Adrian Mannarino in the first round in Cincinnati where Venus is defending semifinal points from a year ago.
Tsitsipas broke and forced a second-set tiebreaker, where Nadal steadied the ship and elevated his game once again.
"I think the biggest thing I was impressed with was definitely his mentality", Anderson said about Tsitsipas.
Tsitsipas beat the fourth-seeded Anderson after topping seventh-seeded Dominic Thiem, ninth-seeded Novak Djokovic and second-seeded defending champion Alexander Zverev to reach his first ATP World Tour Masters 1000 semifinal.
"He was [once] normal like all of us, and he managed to become this beast, this monster that he is today". He'll crack the top 15 in the world rankings next week. "I need to work much more and hopefully I can reach his level one day".
Earlier, a vocal crowd of local flag-waving Greek fans had sang "Happy Birthday" to the 20-year-old Tsitsipas as Nadal reflected on a successful campaign.
"You make me play better and better every time we play", Halep said of Stephens.
In the doubles final, Finland's Henri Kontinen and Australia's John Peers topped Raven Klaasen of South Africa and Michael Venus of New Zealand 6-2, 6-7 (7), 10-6.