Serena Williams wept as back pain forced her out of the WTA Toronto final after just four games on Sunday, handing the title to home hope Bianca Andreescu.
Andreescu, 19, was up 3-1 with a break of serve when 23-time Grand Slam champion Williams found she couldn't continue.
When it was clear that Williams would retire, the young Canadian 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.
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.
"I'm sorry I couldn't do it today," she told the crowd, her voice cracking. "I tried, but I couldn't do it.
"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 -- when she was already pregnant.
Since returning to competition 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 1969.
"I know how it is to pull out of tournaments and be injured, it's not easy," she 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, who beat world number five Kiki Bertens and third-ranked Karolina Pliskova on the way to the title, now owns a stunning 7-0 record against top-10 players in her career.
She's projected to break into the top 15 in the world rankings.