But the second seed credited a new-found self belief for pulling off a comeback quarter-final win Osaka admits she wouldn't have been able to achieve just six months ago.

World No.5 Osaka of Japan became the first woman into the Brisbane semi-finals when she overcame eighth-seeded Latvian Anastasija Sevastova 3-6 6-0 6-4.

World No.11 Sevastova won both her clashes with Osaka in 2018.

And the 28-year-old looked like making it three straight when she jumped to a 2-0 lead before claiming the first set in 33 minutes.

Enter Osaka's coach Sascha Bajin.

Called over by a concerned Osaka ahead of the second set, Bajin's calming influence first settled her down - then clearly fired her up.

The 21-year-old came out swinging, claiming the second set in just 20 minutes before overcoming late resistance from the US Open semi-finalist.

"He (Bajin) basically said not to be so worried. It was just acknowledging that she was playing really good, and there was nothing that I could really do about that," Osaka said.

"And that I had to wait for my chance, and I think that's what I did really well in the second set."

That's an understatement.

Osaka conceded just five points in the second set as she powered her way to a turnaround win that would have no doubt made rivals sit up and take notice ahead of the Australian Open.

Japan's first grand slam winner had enjoyed a stellar 2018, rising from world No.68 to earn a season-ending top five ranking.

She has also earned plenty of self belief ahead of a semi-final clash with Ukraine's Lesia Tsurenko, who edged out Estonia's Anett Kontaveit 7-5 6-3 in Thursday's final match on Pat Rafter Arena.

"Maybe six months ago I wouldn't have been able to do that (come back)," Osaka said.

"I feel like right now I'm really confident in myself ...so I'm not that scared or threatened.

"I'm not sure if I would have had the same feeling six months ago - but six months ago I hadn't won the US Open.

"But now I sort of know what could potentially come next."

Many would be predicting a second grand slam may be next after the stunning turnaround that ensured Osaka has made at least the semi-finals in four of her last five tournaments.

Osaka wasn't so sure when asked about Melbourne favouritism.

"You guys (media) might pick me as a (Australian Open) favourite but I don't know, there's so many other players," she smiled.

Australian Associated Press