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.
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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

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"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.
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