The former World No. 1, trailing Kanepi 7-6(4), 3-0, had to call it quits as she couldn't pull through to the end of the match. In a competitive first set featuring two medical timeouts, Osaka rallied from a break down and saved a set point in the 12th game.
However, Kanepi converted her second set point in the ensuing tie-break to streak ahead. The Estonian veteran then reeled off three straight games before Osaka threw in the towel.
Explaining her injury, the 24-year-old said that she felt something in her back from the start of the match.
“I felt my back from the start of the match, and despite trying my best to push through it, I just wasn’t able to today," said Osaka. "I’d like to pay credit to Kaia for playing well and want to wish her all the best for the rest of the tournament.”
2022 Western & Southern Open WTA Entry List headlined by Swiatek, Osaka, Raducanu, Fernandez, Gauff and Serena Williams
It's unclear if she'll be fit in time to play at the Western & Southern Open next week and the US Open later this month.