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."