(CNN)In a stunning result, US Open defending champion Naomi Osaka lost her third-round match to Canada's Leylah Fernandez in three sets, as the world No. 3 and four-time major champion uncharacteristically showed signs of frustration on the court and expressed doubts after the match.
Naomi Osaka considering taking another break from tennis after US Open loss
"I honestly don't know when I'm going to play my next tennis match," Osaka said through tears in a press conference. "I think I'm going to take a break from playing for a while."
Osaka, who was leading by a set, broke Fernandez for a 6-5 lead in the second set. But when serving for the match, Osaka couldn't close it out. During the tiebreak, Osaka slammed her racquet to the court.
Fernandez, 18, would win the tiebreak to force a final set, and then the world No. 73 broke Osaka to open the third. Not long after that, Osaka hit a ball into the crowd and was hit with a code violation for ball abuse.
Fernandez went on to win the match 5-7, 7-6(2), 6-4.
The Canadian, who turns 19 on Monday, will next face No. 16 seed and three-time major champion Angelique Kerber of Germany.
Fernandez -- who played a brilliant match -- gave credit to the crowd in her on-court interview, as the Canadian had tremendous support from those in attendance.
"It felt so energetic," the Canadian teen told ESPN. "The fans were amazing. They were so loud. They helped me so much throughout the second set and then the third. I'm just so happy to be on the big stage finally."
In her on-court interview after the win, she said she knew "from the very beginning" that she could beat the defending US Open champion.
"Right before the match I knew I was able to win," she said.
Osaka expressed regret after the match regarding her emotions shown on the court.
"Yeah, I'm really sorry about that. I'm not really sure why. Like, I felt like I was pretty -- I was telling myself to be calm, but I feel like maybe there was a boiling point," she said.
"I feel like for me recently, like, when I win I don't feel happy," she said. "I feel more like a relief. And then when I lose, I feel very sad. I don't think that's normal."
In May, Osaka withdrew from the French Open after being fined $15,000 for refusing to speak to the media at Roland Garros, and chose to not participate in this year's Wimbledon Championships.
Fernandez was asked about Osaka losing her composure during the match.
"Honestly I wasn't focusing on Naomi," Fernandez said. "I was only focusing on myself and what I needed to do."
As for turning the match around when she broke back against Osaka to level the second set at 6-6, Fernandez said, "I guess I wanted to stay on court a little bit longer, and I wanted to put on a show for everybody here. One hour was just not enough for me on court."