The Japanese world No.4 brought her parents Tamaki Osaka and Leonard Francois to Melbourne Park to watch her defend her Open crown, with their visit cut short with her shock defeat by American teenager Coco Gauff.

Let down by 30 unforced errors, Osaka fell 6-3 6-4 in just over an hour in a result she described as a "reality check".

"This one hurts a little bit more," the 22-year-old said.

"I feel bad for Wim (coach Wim Fissette), for my entire team.

"We came here to win the tournament and I'm sort of like the vessel that everyone's hard work is put into and I wasn't able to do what I was supposed to do.

"My dad and my mum were here, so that kind of hurts more."

The pair met in the second round of the US Open just a few months ago, with Gauff then only managing to win three games.

Osaka admitted she may have under-estimated the youngster, who was playing in her first Australian Open.

"Yeah, I guess so, you could say that," the former world No.1 said.

"She was more aggressive and her serve is way better than when I played her last year.

"It's tough - you don't want to lose to a 15-year-old- but I guess that's for me a reality check.

"I don't like losing to people that are younger than me ... I took this very personally."

The 2018 US Open champion bowed out in the fourth round last year in New York when defending that title and said she was still learning how to deal with the status of a champion.

"I just feel tight playing here a little bit because of the defending thing," Osaka said.

"I feel like there are moments where I can handle them and then there are moments like this where I get overwhelmed and I don't really know what to do in the situation.

"I think I just have to keep learning but I don't know - this is a very new situation for me to be in.

"I don't really have the champion mentality yet - someone that can deal with not playing 100 per cent (and still win).

"I've always wanted to be like that, but I guess I still have a long way to go."

Australian Associated Press