Naomi Osaka admitted to buckling under the pressure of having to defend her Australian Open title, saying she was still searching for the "champion mentality", following her third-round upset by American teenager Coco Gauff on Friday.
The Japanese third seed, who beat Petra Kvitova to win the title at Melbourne Park last year, appeared out of sorts in her loss against 15-year-old Gauff, racking up 30 unforced errors to just 17 winners.
Osaka's tame performance on Rod Laver Arena came less than four months after she beat Gauff easily 6-3 6-0 in the same round at the U.S. Open.
"I don't know. I just feel tight, like, playing here a little bit because of the defending thing," Osaka told reporters.
"I don't really have the champion mentality yet, which is, like, someone that can deal with not playing 100%, you know. And I have always wanted to be like that, but I guess I still have a long way to go."
The loss marks the second Grand Slam in a row where Osaka has failed to retain her title, after losing to Swiss Belinda Bencic in the fourth round at the U.S. Open last year. Osaka had won her first major final in New York in 2018, beating six-times champion Serena Williams.
Osaka said she was trying to get back into the mindset of winning tournaments rather than focusing on her performance in previous seasons.
"I think that's easier than thinking about defending," she said.
Osaka, who was hailed as the future of women's tennis after her double Grand Slam success, also acknowledged that the loss to the teenage Gauff stung.
"You don't want to lose to a 15-year-old, you know. But I guess that's for me, like, a reality check. It doesn't really matter the age of the opponent," she said.
"Of course she deserves to be here. She played her matches. I just have to work harder."