Osaka's US Open title defence came to a remarkable end after imploding in Saturday's (AEST) surprise 5-7 7-6 (7-2) 6-4 loss to teenage sensation Leylah Fernandez.

Up a set and serving for the match at 6-5 on Arthur Ashe Stadium, Osaka lost her cool and composure after throwing her racquet three times in an unsuccessful second-set tie-break.

Amid boos in New York, Osaka was also warned after hitting a ball into the crowd and while the four-time Major champion tried to dig herself out of a hole, she crashed out in incredible fashion.

It comes after a difficult couple of months owing to mental health concerns and a result of "long bouts of depression" since winning the 2018 US Open.

Osaka withdrew from May's French Open having won her first-round match, after she was fined and threatened with further punishment – and possible expulsion from the grand slam – for skipping obligatory media duties.

The Japanese star subsequently pulled out of Wimbledon before returning for the Tokyo 2020 Olympic Games, though she suffered a surprise loss on home soil and was reduced to tears during a news conference in Cincinnati.

During an emotional post-match news conference, Osaka said: "I'm going to say what we said, I think, like, in the hallway. How do I go around saying this?

"I feel like for me recently, like, when I win I don't feel happy. I feel more like a relief. And then when I lose, I feel very sad. I don't think that's normal. I didn't really want to cry.

"I feel like… this is very hard to articulate. I feel like I'm kind of at this point where I'm trying to figure out what I want to do, and I honestly don't know when I'm going to play my next tennis match [tearing up]. Sorry."

Osaka, who was bidding to become the first woman to defend the US Open since Serena Williams in 2014, added: "I think I'm going to take a break from playing for a while."

Typically reserved and quiet, Osaka was uncharacteristically frustrated on court, the 23-year-old immediately left the court and emerged with a towel over her head before the start of the final set.

On her outburst, Osaka said: "I'm really sorry about that. I'm really sorry about that. I'm not really sure why.

"I was telling myself to be calm, but I feel like maybe there was a boiling point. Like normally I feel like I like challenges. But recently I feel very anxious when things don't go my way, and I feel like you can feel that. I'm not really sure why it happens the way it happens now.

"It's basically why. You could kind of see that. I was kind of like a little kid."