News of the pending retirement of Serena Williams overshadowed proceedings at the final tune-up event, WTA Toronto, for the year's final Grand Slam but there were no shortage of first-round upsets for fans to enjoy, including one by local Bianca Andreescu.
Twice U.S. Open champion Osaka, in only her second tournament back from an Achilles injury, battled on gamely before retiring while trailing Kaia Kanepi 7-6(4) 3-0.
"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.
Toronto-born Raducanu conceded a tight first set to Camila Giorgi but looked on course to level up the contest when she took a 2-0 lead in the second. But the Italian defending champion stormed back to rattle off the last six games with a mix of solid returns, net play and movement, easing into the second round 7-6(0) 6-2.
"I think it was a really good match, to be honest," said ninth seed Raducanu. "I just need to get better at dealing with players who play as quick as she does."
Andreescu later delighted a packed house with a 7-6(5) 6-4 victory over Daria Kasatkina in an evening match that left the Russian world number nine, fresh off a title win in San Jose, clearly disgruntled.
The Canadian, who took the Toronto title in 2019 before winning the U.S. Open, twice called out the trainer, including during the first-set tiebreak, before prevailing in a topsy-turvy match on centre court.
"I was honestly a mess," said the 53rd ranked Andreescu. "I'm so sorry for my attitude, I really do not feel well, I don't know what it was ... maybe I ate too much before the match."
There were also first-round wins for 10th seed Coco Gauff, China's Zheng Qinwen, Belgian Elise Mertens, Spain's Sara Sorribes Tormo and Australia's Ajla Tomljanovic.
Greek third seed Maria Sakkari stamped out any chance of another upset in the final match of the day, fighting off a comeback from 2017 U.S. Open champion Sloane Stephens to take a 6-2 4-6 6-2 victory in a second round tie.
The news that six-times U.S. Open champion Williams would soon be hanging up her racket dominated talk among players and fans around the complex.
"She changed the sport and she did a lot and now she can bow down," spectator Natalie Chapusette said of the 40-year-old.
"It is good news for her, but not for the fans. But if you see the human behind the sportsperson, she deserves it."
Kanepi, however, was unmoved by the news.
"I took it very calmly," she told reporters. "Because, you know, players, they retire at a certain age. I think it's normal."
