A laser-focused Ashleigh Barty dismantled American giant-killer Amanda Anisimova on Sunday to battle into an Australian Open quarter-final against Jessica Pegula.

The steely-eyed Australian world number one won 6-4, 6-3 on Rod Laver Arena to halt the charge of the unseeded American who stunned defending champion Naomi Osaka in round three.

Top-seed Barty, who is aiming to become Australia's first women's champion in 44 years, will now face 21st-seeded American Pegula following her upset of Greek fifth seed Maria Sakkari 7-6 (7/0), 6-3 to make her second straight quarter-final in Melbourne.

"She's an incredible athlete, she's an incredible competitor. One of her best attributes is she is able to turn up point after point," said Barty of Anisimova.

"She's got an exceptional game that puts you under the pump right from the get-go and it was nice to hold firm tonight and bring the points back into my patterns more regularly and the big ones when it mattered most."

It was a rematch of the pair's memorable 2019 French Open semi-final.

Barty was staring down the barrel of defeat then after a first-set collapse, but fought back to win the match and eventually the title.

It proved to be a turning point in her career and she has now been  world number one for 112 consecutive weeks.

Anisimova was only 17 at the time and fell down the rankings following the death of her father soon afterwards, but she has been rejuvenated this year.

Barty missed a break point opportunity on Anisimova's opening serve then was forced to save one on her own to keep the scores level-pegging at 2-2 after a rare double fault.

A net volley set up two more break points on the next Anisimova serve as Barty again probed for an opening.

The relentless pressure paid off with some powerful ball-striking working a triple break point and this time she capitalised to move 4-3 ahead.

She wrapped up the set in 37 minutes, winning 84 percent of her first serve points to Anisimova' 67 percent.

But Anisimova then did what no one else had managed this tournament -- she broke Barty to end an incredible run of 63 consecutive service holds this year.

It was in vain as the Australian immediately broke back utilising her slice and loopy forehands, then broke again for 4-3 and with Anisimova wavering she powered home.

After shutting down her 2021 season after the US Open, Barty has been dominant this year. She returned at the Adelaide International to win the title and has improved in Melbourne.