The steely-eyed world No.1 had to fight off eight break points in a tense first set before denying the free-hitting Czech 7-6 (8-6) 6-2 in a rematch of their 2019 quarter-final at Rod Laver Arena.

The first Australian to make the Open semi-finals since Wendy Turnbull in 1984, Barty will play American 14th seed Sofia Kenin on Thursday for a spot in Saturday night's title decider.

"It's been absolutely incredible. I knew that I had to bring my absolute best today against Petra. That first set was so crucial," Barty said.

"I just wanted to try to make the most of it and it was really good to get a quick start in the second."

With Kvitova typically coming out swinging, Barty found herself under pressure from the off.

The reigning French Open champion had to fight off a break point in the opening game as Kvitova pounced on anything short.

She couldn't hang on a second time, though, with Kvitova nabbing the first break in the third game - only to hand it straight back by dropping serve to love with a succession of unforced errors.

Still on the back foot as Kvitova dictated proceedings with her ultra-aggressive first-strike tennis, Barty did brilliantly to save three more break points in the seventh game to hold for 4-3.

Barty's extraordinary defensive skills were on full display in the vital first-set tiebreaker.

Serving at 2-3, Barty had no right to win the longest rally of the match - a 22-shot exchange that Kvitova eventually surrendered with a backhand into the net after the top seed wore the Czech down with her relentless retrieving.

Scrapping desperately to stay in it, Barty saved a set point on her own serve and her perseverance was rewarded when Kvitova floated a backhand long to concede the tense first set after 69 minutes.

With the wind in her sails, Barty stormed to a 4-0 lead in the second to seize total command.

Kvitova briefly threatened a comeback after breaking Barty to reduce the deficit to 4-2.

But the home hope was not to be denied, holding her nerve to seal victory after one hour and 44 minutes.

Her milestone 250th tour-level victory, and 100th on hard courts, vaulted the 23-year-old Barty to within one tantalising win of following Turnbull again as the first Australian women's finalist in Melbourne since 1980.

Barty has won four of her five clashes with Kenin, including their fourth-round showdown last year in Paris.

"She's developed a lot over the last 12, 18 months. She's a great ball striker who wants to be inside the court taking the ball either way," Barty said.

"It's about me getting my running shoes on, bringing variety and trying to bring the match on my terms as much as possible."

Australian Associated Press