A lower back injury forced Japan's Naomi Osaka to retire from her first-round match at the National Bank Open on Tuesday in Toronto.

Estonia's Kaia Kanepi led Osaka 7-6 (4), 3-0 when the four-time Grand Slam winner had to pack it in.

"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," Osaka said. "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."

Another successful young star had an early exit Tuesday when ninth seed Emma Raducanu of Great Britain lost 7-6 (0), 6-2 to Italy's Camila Giorgi.

Giorgi, though unseeded, is the WTA 1000 event's defending champion. She overcame six double faults by winning 29 of 41 first-service points (70.7 percent).

Giorgi trailed 2-0 in the second set but won the final six games of the match. That included a stunning 16 straight points from the winning point of the fourth game all the way into the eighth and final game. Giorgi won on her third match point.

"I think it was a really good match, to be honest," Raducanu told reporters, despite the loss. "I think that the level was pretty high. Especially in the first set. And Camila is a great opponent. ... I just need to get better at dealing with players who play probably as quick as she does."

Coco Gauff and Belinda Bencic avoided a similar fate to Raducanu, each winning in straight sets. Gauff, the 10th seed, beat fellow American Madison Brengle 6-1, 6-3 in 62 minutes. Bencic, the 12th seed from Switzerland, overcame the Czech Republic's Tereza Martincova 6-4, 6-2.

Bencic's reward for advancing will be a second-round date with Serena Williams. The 40-year-old superstar revealed Tuesday in a first-person essay that she planned to begin "evolving away from tennis" and soon retiring from the sport in favor of her family and business ventures.

Other winners Tuesday included American Amanda Anisimova, Shuai Zhang and Qinwen Zheng of China, Ajla Tomljanovic of Australia, Sara Sorribes Tormo of Spain and Elise Mertens of Belgium.

Two primetime matches had yet to be decided. Canadian favorite Bianca Andreescu won her first set over Russian 11th seed Daria Kasatkina 7-6 (5). Third seed Maria Sakkari of Greece was scheduled to play Sloane Stephens in the only second-round match of the day.

--Field Level Media