Canadian teen Leylah Fernandez stuns Naomi Osaka at US Open

Leylah Fernandez stuns Naomi Osaka in third round (0:46)

18-year-old Leylah Fernandez upsets Naomi Osaka in the second round of the US Open to advance to the round of 16. (0:46)

NEW YORK -- Leylah Fernandez, an 18-year-old from Canada ranked 73rd in the world, was confident she could best defending US Open champion Naomi Osaka in their third round match at Arthur Ashe Stadium on Friday night.

Before this year's US Open, Fernandez had never been past the second round of a Grand Slam. Yet her confidence turned out to be well-founded.

"Right before the match, I knew I was able to win,'' Fernandez said after her 5-7, 6-7 (2), 6-4 victory over Osaka.

Osaka had come into Friday with a 16-match winning streak at majors and four career trophies, all on hard courts. But this was Osaka's first Slam tournament since she pulled out of the French Open before the second round to take a mental health break -- she also sat out Wimbledon, before participating in the Tokyo Olympics, where she lit the cauldron -- and maybe the time off was an issue.

Another possible factor in her failure to close things out while serving for the victory against the left-hander Fernandez at 6-5 in the second set: Osaka hadn't competed since Monday. That's because the woman she was supposed to meet in the second round, Olga Danilovic, withdrew because she was sick.

The 23-year-old, who was born in Japan and has lived in the U.S. since she was 3, left the court after seven games to change outfits and put on a yellow dress. She was quite good down the stretch in the opening set. She grabbed 12 of the last 13 points, including the last nine, with a break at love to go up 6-5, and a hold at love with the help of a pair of aces at 112 mph and 114 mph to end it.

Osaka seemed on track for a similar conclusion in the second set, leading 6-5 and serving. But when she sailed a forehand wide, Fernandez had her first break of the match to make it 6-all.

"I guess I wanted to stay on court a little bit longer. And I wanted to put on a show for everybody here,'' Fernandez said. "One hour was not enough for me to be on court.''

And so began Osaka's downward spiral. She fell behind 5-0 in the ensuing tiebreaker, missing shots and displaying her frustration, as she occasionally has done in the past, by flinging her racket. Chair umpire Alison Hughes did not sanction Osaka then, although later a warning was issued for hitting a ball into the stands.

"Honestly, I wasn't focusing on Naomi,'' Fernandez said. "I was only focusing on myself and what I needed to do.''

More to the point, Osaka was not at her best. She left the court with a white towel draped over her head after the second set and never really got her game going the rest of the way.

Fernandez certainly had something to do with that, particularly because of her serve. She won 18 of 19 first-serve points -- and never faced a break point -- in the third set.

Fernandez's knee-to-the-ground, quick-redirect style at the baseline is reminiscent of another lefty, Angelique Kerber, a three-time Grand Slam champion who won the 2016 US Open. And who just so happens to be the next opponent for Fernandez.

"I'm going to put on a show like I did tonight,'' Fernandez said, "and we're going to see how it goes.''

The Associated Press contributed to this report.