23-time Grand Slam champion Williams was suffering an upper back injury and the legendary American was unable to continue after falling behind an early break in the final.
Andreescu is now a bonafide competitor, but whatever sting that took from winning her hometown tournament will wear off eventually when she reflects on an unbelievable week of tennis during which she showed exactly why she's a legitimate threat on the tour - with limitless potential. In fact, Williams turned professional five years before the birth of Andreescu.
Williams delivered a tearful speech after the game, saying she was sorry she couldn't go on. Prior to making her return in Toronto, the Mississauga native's last WTA Tour action was at the French Open.
"It's just about going out there and playing your great tennis".
"Are you ok? What happened?"
There was the memorable 2012 run that saw Aleksandra Wozniak beat former world No.1 Jelena Jankovic and became the first Canadian in 20 years to reach the quarter-finals.
As Williams sat dejected in her chair, Andreescu, who is no stranger to injury setbacks, went over to console her. "I think it pumps me up even more".
"It's not easy, that's for sure", said Andreescu, who was still shivering from a lengthy ice bath as part of the treatment on her groin.
It was a jarring and shocking end to a match so many were anticipating.
Serena later explained to the Rogers Cup crowd that she simply couldn't tough it out that day, noting that she had tried.
Williams was the first to address the crowd. "I'm just going to try and stay in the present moment".
The crowd cheered as Williams continued.
Making a full return in front of the home fans, Andreescu gave her 120% to overcome all the rivals before the final, ousting three top-10 players in total to improve her tally against them to a ideal 7-0 at such a young age. And thanks to my team. She had beaten Naomi Osaka, who she faced in last year's US Open final, in the quarter-finals. "It's not easy", Andreescu began. I'm the first Canadian to make the final and win this tournament since 1969. "You are truly a champion", the Canadian added.
"Within my lifetime there hasn't been that many - I'm 34 and there hasn't been many Canadians up there in the top rankings", says recreational tennis player Luke Naka.
The two showered each other with respect. She provided comment during a press interview after the match on August 11.
It was a dream matchup for tournament organizers and fans alike - the breakout star and hometown hero versus the icon of her sport.
Williams was certainly impressed, calling the Canadian an "old soul".