Andreescu then consolidated her break to move 3-1 up at which point Williams immediately moved to her chair where she broke down in tears.
Of course, nobody will complain about the result, which saw 19-year old Canadian sensation Bianca Andreescu cap off a week that saw her beat three top-10 players en route to becoming first Canadian singles victor at this event in 50 years.
Andreescu has six career wins over top-10 players, all of which came this year. Her world ranking will raise from 27th to at least 19th on Monday. Her previous high was 22nd.
She won in her first tournament since the French Open in May, when a right-shoulder injury forced her to stop playing.
Andreescu had been on the court more than any other player at this year's Rogers Cup at 10 hours, 54 minutes heading into the match against Williams.
Williams has now featured in four finals since giving birth to her first child in September 2017, failing to win any of them.
After the chair umpire announced Williams's decision, the former world No. 1 started to cry.
Andreescu advances to 3rd round at Rogers Cup with marathon three-setter
It's going to be interesting to see how Andreescu responds physically given the wear and tear to get to this point. I've been off for seven weeks and being able to win these tough matches after that period off means so much.
"She definitely doesn't seem like a 19-year old in her words, on the court and her game, her attitude, her actions", Williams said, adding that her brief time on the court with Andreescu on Sunday was the highlight of the tournament for her.
Canadians Faye Urban and Vicki Berner played in the 1969 final, with Urban winning the tournament that was played on clay and called the Canadian Open. "I know what it's like to pull out of tournaments and be injured".
It's beginning to feel like Bianca Andreescu might never lose again.
Thank you, guys. I'm sorry I can't do it today, I tried but I just couldn't do it. Bianca, you are a great sportswoman. "If I can do it, if Serena can do it, anyone can do it if you just believe in yourself", Andreescu said.
Later Friday, American Serena Williams beat Naomi Osaka of Japan 6-3, 6-4 in a rematch of last year's controversial U.S. Open final.
She'll play the victor of the second semifinal between American Serena Williams and Marie Bouzkova of the Czech Republic. On the men's side, Milos Raonic retired after two sets against Felix Auger-Aliassime in a much-anticipated all-Canadian matchup. In 2010, fans in Toronto watched in amazement late into the night when a 217th-ranked Milos Raonic and 329th-ranked partner Vasek Pospisil teamed up in doubles and beat the team of Novak Djokovic and Rafael Nadal - then-ranked No. 1 and No. 2 in the world. "But when things happen like that I just try to focus on the tactics and forget about the pain, but it's not easy".