Bianca Andreescu (left) consoles Serena Williams during the Rogers Cup tennis tournament final on Sunday in Toronto, Canada. Photo: VCG

A tearful Serena Williams saw her US Open preparation thrown into disarray Sunday as back spasms forced her out of the WTA Toronto final after just four games, handing Canadian teenager Bianca Andreescu the title.

Andreescu, 19, was up 3-1 with a break of serve when 23-time Grand Slam champion Williams decided she couldn't continue.

She called for a medical timeout, but within a minute, the umpire announced she was retiring.

"I just knew," said Williams, adding she'd undergone hours of treatment before the match to combat the back spasms she'd first felt in a semifinal win on Saturday night. "I knew I wasn't going to be able to continue."

Andreescu approached her chair, offering words of encouragement and a consolatory hug as Williams tried to fight back tears.

Williams said the spasms started during her come-from-behind win over Czech qualifier Marie Bouzkova on Saturday "and it just got worse."

"Just my whole back just completely spasmed, and to a point where I couldn't sleep and I couldn't really move," she said.

Nevertheless, she wanted to give it a go in the final.

"I don't want to get this far and not at least try," said the 37-year-old, who was eyeing her 73rd WTA title - and her first since the birth of her daughter, Olympia, in September 2017.

Beaten by Simona Halep in the Wimbledon final in July, Williams was playing just her 24th match of a year already disrupted by nagging knee trouble.

Williams hasn't won a title since the 2017 Australian Open.

Since returning to competition after Olympia's birth, she has reached three Grand Slam finals - and she remained optimistic that she would be ready to try to match Australian Margaret Court's record of 24 Grand Slams when the US Open starts on August 26.

"Well, that's the most frustrating part is that I've had this before and it's, like, 24, 36 hours where I'm just in crazy spasm and then it's, like, gone," said Williams, who wasn't immediately sure whether she'd play this week in Cincinnati.

"So that's a little bit frustrating for me because I know that I could play. I just can't play today.

"If it's what typically happens, I will be fine, but I have to wait and see."

For Andreescu, Sunday's events were a bittersweet achievement as she became the first Canadian to win the title since Faye Urban in 1969.

"I know how it is to pull out of tournaments and be injured - it's not easy," Andreescu said, addressing Williams directly during the trophy presentation.

"This wasn't the way I expected to win and for you to go off the court. I've watched you play so many times. You are truly a champion on and off the court."

Andreescu claimed her second WTA Premier level title of the year, having rocketed to prominence in March when she became the first wild card to win at Indian Wells.

"This week has not been easy. I've had many, many tough matches," said Andreescu, who opened the week with an emotional three-set win over compatriot Eugenie Bouchard and played three more three-setters.