Williams had enjoyed an impressive week, beating the likes of Naomi Osaka, to reach the final in Toronto.
And the 23-time Grand Slam champion was meeting the hometown favourite Andreescu for the first time, but to the disappointment of the sell-out crowd, the eagerly anticipated final came to an abrupt end.
Andreescu broke early in the first set to lead 2-1 and then consolidated the break after an unforced error from Williams.
But after losing the game, Williams tried to hold back the tears asshe walked to her chair and called for a trainer.
After talking to the physio it was announced Williams would no longer be able to continue and Andreescu walked over and embraced her distraught opponent.
A tearful Williams said in the trophy ceremony: “I’m sorry I can’t do it today, I tried but I just couldn’t do it.
“Thank you, Jehovah for letting me come out here. Bianca, you’re a great sportswoman. Thanks to my team.
“It’s been a tough year, but we’ll keep going.”
With the victory, Andreescu becomes the first Canadian woman to win the final of the Rogers Cup since 1969.