Barty was listed to be the third match on Rod Laver Arena but Simona Halep's record-equalling win over Lauren Davis which went for almost four hours forced a change of schedule, as tournament officials sought to avoid the prospect of Roger Federer taking to court after midnight.

Shunted to Margaret Court Arena, Barty struggled to deal with Osaka's power and was ousted 6-4, 6-2.

"Of course, I am happy. Winning this match is one of my goals ... but it's not like I have won the tournament. I am not satisfied yet," Osaka said.

"I feel the way tennis is now, certain people have chances (to win) and I would like to be one of those people."

Barty had been able to conjure victory after falling behind a set in her opening two matches but, on this occasion, she was unable to rebound against the world No.72.

While delighted to have won, Osaka said it was disappointing to have knocked the last Australian woman out of the tournament.

"She (Barty) has been doing super well. She is the best Australian, it kind of sucks - I know they (fans) would want someone in the final. I am just glad they didn't boo me - I learned a lot," she said.

Osaka, 20, broke once in the first set and that was enough to take control, and opened by blunting Barty's serve twice in the second set to surge to a 4-0 lead - and all but seal victory.

Barty had never advanced through to the round of 16 at her home Grand Slam - she also fell at this hurdle last year - and will have to wait at least another season to do so.

The Queenslander, who now calls Melbourne home, did not settle well and could not dictate play with her forehand.

Osaka, who had some major scalps last year, is seen as one of the WTA's stars on the rise - and she proved that against Barty, who had a wonderful 2017 where her ranking rose to No.17 in the world and she had multiple wins against top-10 opponents.

Barty raised her intensity in the second set and would hold serve in the fifth match but Osaka was sublime, her attacking and power game all but wiping her opponent from the court.

"I wasn't able to get into too many of her service games. I had a sniff at a couple games - 15-30s, 30- all points - but she served great and played the big points well," Barty said.

"I'm obviously very happy with my first couple matches. Even tonight, I don't think I did a hell of a lot wrong. I think if Naomi plays like that, she beats a lot of players."

There was an unlikely speck of hope when Osaka, serving for the match, dropped the opening two points. But Osaka's thumping serves ensured she shrugged off trouble - and she finished off the match with a thundering ace, claiming the second set in 35 minutes.

Osaka said diligent off-season training had helped her prevail.

"We did a lot of fitness in the off-season. I was mainly focusing on consistency and mindset," she said.

Osaka said she had worked on being stronger in mind, pointing to the several occasions when she had lost the first two points on her serve but had regrouped to take the game.

Channel Seven commentator Rennae Stubbs, a former Australian star, said the change of court had benefited Osaka, for Barty had claimed her two wins on Rod Laver Arena.

"Ash has handled herself so beautifully on Rod Laver. If it is anything about moving the court, it certainly helps Osaka more than Ash," Stubbs said.

Barty said the court change had not impacted on the result.

"I had phenomenal support in Margaret Court Arena, as well. It's probably a little bit of a late change for everyone to know but it was still a very full crowd. I enjoyed playing out there. I would have loved to have come off as a winner, but it wasn't meant to be tonight," she said.

Osaka said the change of court had not disrupted her routine.

"No, because I (had been) playing on court 10 and court 7. Like any court with a stadium ... I'm always happy to play on it," she said.

Halep's epic victory over Davis meant the men's clash between Alexander Zverev and Hyeon Chung on Rod Laver Arena did not begin until late afternoon. Had Barty's clash gone ahead, it would have meant the prime-time matches between Maria Sharapova and Angelique Kerber, with Federer and Richard Gasquet to follow, would have begun late in the evening, impacting on broadcast ratings.

Barty, a former Wimbledon junior champion, won her maiden singles crown last year, having not long returned from a 16-month hiatus where she played cricket in the Women's Big Bash League. She also knocked over Grand Slam champions Angelique Kerber, Garbine Muguruza, Venus Williams and Jelena Ostapenko last year.

Despite her home summer coming to a premature end, there is much for Barty - who last weekend reached the final of the Sydney International - to look forward to this year.

"It's really about maintaining my spot in the top 20, solidifying my spot, continuing to play good tennis, do the right things on and off the court," she said.

"(I) would love to make the second week of a slam. We're close. We'll keep chipping away.

"I think the next couple months are busy. We have Fed Cup before moving through the Middle East, through Indian Wells, Miami, then Fed Cup again. That's sort of a pretty standard schedule for most players."