Toronto -Tearful Serena Williams saw her US Open preparation thrown into disarray on 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 found she couldn't continue.
She called for a medical timeout, but within a minute the umpire announced she was retiring.
Andreescu then approached her chair, offering words of encouragement and a consolatory hug.
An on-court microphone caught Williams' mention of back spasms, with the official cause of her retirement given as an upper back injury.
"I'm not a crier, but, thank you guys," Williams said, her voice cracking during the trophy ceremony.
"I'm sorry I couldn't do it today. I tried but I just couldn't do it."
It was a big blow for the 37-year-old who was closing in on her 73rd WTA title - and her first since the birth of her daughter, Olympia, in September of 2017.
Beaten by Simona Halep in the Wimbledon final in July, Williams was playing just her 24th match of year disrupted by nagging knee trouble.
"Bianca you're a great sports person, woman. Thanks to my team, it's been a tough year, but we'll keep going."
The outcome left Williams in search of her first title since the 2017 Australian Open.
Since returning to competition after Olympia's birth she has reached three Grand Slam finals but has been unable to equal Australian Margaret Court's record of 24 major titles.
It remains to be seen if her latest injury will threaten her chances of trying again for number 24 at the US Open, the final Grand Slam of the year that starts at Flushing Meadows on August 26.
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.
Andreescu, who beat world number five Kiki Bertens and third-ranked Karolina Pliskova in Toronto, now owns a 7-0 record against top-10 players in her career and is projected to break into the top 15 in the world rankings on Monday.
"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.
It was her first tournament back since a right shoulder injury sidelined her at the French Open.
"What I've been through the past two months has not been easy," Andreescu said.
"I kept telling myself 'never give up.'"