The 37-year-old - who was seeking to win her first title since returning to competition in 2018 after the birth of her daughter - was reduced to tears courtside after suffering back spasms.
The American withdrew when a break down in the opening set, and the 23-time Grand Slam champion said afterwards:
I'm sorry I couldn't do it today, I tried. It's been a tough year, but we'll keep going.
Australian Open
‘It was really painful’ – Cornet reveals 13 years of hurt in emotional on-court interview
Andreescu, who won her first WTA title at Indian Wells earlier this year, comforted Williams after the withdrawal - and she becomes the first Canadian to win the Rogers Cup women's singles title since 1969.

Bianca Andreescu

Image credit: Getty Images

"Serena, you made me cry. I know how it is to pull out of tournaments, it's not easy," she said.
"This wasn't the way I expected to win. You are truly a champion. I've watched you win so many times, you are truly a champion on and off the court.
"I'm speechless right now. This has been a dream come true."

Williams is a three-time Rogers Cup champion

Image credit: Getty Images

It brought an unexpected end to this year's Rogers Cup, with Williams retiring for only the second time in a singles final during her career.
The previous retirement also came at the Rogers Cup in 2000 - the same year that Andreescu was born.
Williams will hope to win her 24th Grand Slam title when the US Open starts on August 26.
Australian Open
When is the Australian Open draw? Who could Djokovic face?
12/01/2022 AT 13:28
Australian Open
Australian Open 2022 - Who's playing, when's the draw, what's the schedule
10/01/2022 AT 14:15