Toronto (Canada): A 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 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 semi-final win on Saturday night. "I knew I wasn't going to be able to continue."