Four-time Grand Slam champion Naomi Osaka, wiping away tears after a stunning US Open upset loss on Friday, said she will take an indefinite break from playing tennis.

"I honestly don't know when I'm going to play my next tennis match," Osaka said, choking up and wiping away tears. "I think I'm going to take a break from playing for a while."

With that, the defending US Open champion put on her Covid-19 facemask, rose and ended the news conference after losing to Canadian teen left-hander Leylah Fernandez 5-7, 7-6 (7/2), 6-4.

The world number three from Japan, who withdrew from the French Open and skipped Wimbledon over mental health issues that she said were exacerbated by speaking after matches, declined a chance to end the session before saying she planned to take a break.

"How do I go around saying this?" Osaka asked aloud. "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, but basically I feel like..."

After saying she wanted to continue, Osaka, said: "This is very hard to articulate. Basically I feel like I'm kind of at this point where I'm trying to figure out what I want to do."

Then she announced her tennis break.

Osaka said she thought she served well against Fernandez, but overall she added: "I didn't play that well. Like I didn't move that well at all. It's kind of to be expected sometimes."

Asked about trouble against left-handers, Osaka said, "I can't even tell you how it feels to return it because I don't think I could have returned a ball against a righty today either. I'm pretty sure my return stats were really horrendous.

"It wasn't like she was serving bombs, so I'm not really sure what to say."

Osaka had not played a match since Monday's opener after getting a walkover in the second round.

"I think I would have preferred to play a match. I've never had a walkover in a Grand Slam, so that was definitely a really weird feeling," Osaka said.

"I'm honestly not sure if I feel like I've taken a step today or this tournament. I feel like I'm not really sure what I can say about how I played just now."

- Maybe a 'boiling point' -
Osaka's break from tennis, with a potential title defense coming early next year at the Australian Open, came on a night where she slammed her racquet to the court several times after missing a chance to serve out for the match in the second set.

"I'm really sorry about that," she said. "I'm not really sure why. I was telling myself to be calm, but I feel like maybe there was a boiling point.

"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."

She compared it to a child's temper tantrum.

"I'm not really sure why it happens the way it happens now," Osaka said. "But, yeah, it's basically why. You could kind of see that.

"I was kind of like a little kid."

Asked if the trouble might have come from Fernandez's serve or the tension of the moment, Osaka indicated it was other issues she was dealing with that caused the meltdown.

"I don't think it was her serve because I've been able to return pretty well against people that served better," Osaka said. "I don't think it's the occasion because I've been in this situation before.

"I guess we're all dealing with some stuff, but I know that I'm dealing with some stuff."

Aggressive Alcaraz delivers dream shocker over Tsitsipas
Carlos Alcaraz dug deep for the best tennis of his career when it mattered most and the 18-year-old Spaniard was rewarded with a dream upset of third-ranked Stefanos Tsitsipas.

Alcaraz reached his first Grand Slam fourth round by upsetting the Greek star 6-3, 4-6, 7-6 (7/2), 0-6, 7-6 (7/5) on Friday at the US Open.

"I have not words to explain how I'm feeling right now," Alcaraz said. "I just don't know what happened out there in the court. I can't believe I beat Stefanos Tsitsipas in an epic match.

"For me, it's a dream come true."

Alcaraz is the youngest man in the US Open last 16 since 17-year-old American Michael Chang in 1989 and at any Slam since Ukraine's Andrei Medvedev in the 1992 French Open.

He became the youngest man to beat a top-3 player at the US Open since the rankings began in 1973.

"In the beginning of the fifth set, I had to play really aggressive, the best tennis I've played. Was really tough for me," Alcaraz said.

"I didn't give up. I believed in me in the last point. He was serving really well. I had to be aggressive until the last point. I did it. Was really good."

After being blanked in the fourth set, he out-battled Tsitsipas in a tension-packed final set.

"One hundred percent he can be a contender for Grand Slam titles," Tsitsipas said. "He has the game to be there.

"In the beginning of the first set, (he) came really strong. Ball speed was incredible. I've never seen someone hit the ball so hard. Took time to adjust."

Alcaraz was even more impressive to the French Open runner-up down the stretch.

"I've never seen someone play such a good fifth set," Tsitsipas said. "I didn't expect him to raise his level so much, especially after having lost the fourth set this way. He was a completely different player.

"He dealt with (pressure) really well. That's something that's going to reach the top."

After firing a forehand winner on his third match-point chance of the deciding tie-breaker, Alcaraz collapsed onto the court in joy, thinking of his earliest supporters in his Spanish hometown.

"When I fall into the court at the end of the match I thought my family and my friends, other people who was supporting me in Murcia, all my team also supporting me from the academy," Alcaraz said.

"I thought (of) every person that was supporting me from Murcia from the beginning of this history when I was a kid and everything. Was amazing."

Alcaraz was the clear crowd favorite at Arthur Ashe Stadium, roars rising for deserved dazzling shotmaking.

"It surprised me, really. The crowd was behind me all the time, supporting me, pushing me up in every moment," he said.

"The crowd was important for me. I think without the crowd I couldn't have the opportunity to play a great fifth set and beat Stefanos. The crowd was really amazing. I really loved it."