Melbourne (AFP) - Defending champion Madison Keys revelled in the Melbourne heat Saturday to set up a last-16 showdown with fellow American Jessica Pegula at the Australian Open, where Novak Djokovic hopes to take another step towards a record 25th Grand Slam title.

Players, fans and tournament organisers are braced for highs of 40C in the late afternoon, which could see play suspended.

Matches started an hour earlier than usual because of the heatwave, and Keys had no intention of hanging about.

She dismissed former world number one Karolina Pliskova of the Czech Republic 6-3, 6-3 on Rod Laver Arena in a statement victory that took just 75 minutes.

After admitting to being nervous and timid in embarking on her title defence, this was more like the Keys that stunned Aryna Sabalenka in the final a year ago.

The 30-year-old Keys said that being based in Florida meant she was well used to baking temperatures.

“I am feeling really good and I was excited for the heat today, and ready for it,” she said.

The ninth seed faces good friend and compatriot Pegula for a place in the quarter-finals.

The sixth-seeded Pegula powered past unseeded Russian Oksana Selekhmeteva 6-3, 6-2 in 66 minutes.

“Nice to be first out on court and not a super long match and didn’t need to kill myself in the heat today,” she said.

Another title contender, Amanda Anisimova, made light work of fellow American Peyton Stearns in winning 6-1, 6-4.

Anisimova has emerged as a major force in the past year, reaching the finals at Wimbledon and the US Open.

Multiple major winners Iga Swiatek and Naomi Osaka also both feature on day seven at Melbourne Park.

- Djokovic history bid -

The Serbian great Djokovic faces 75th-ranked Dutchman Botic Van de Zandschulp for a place in the last 16.

The 38-year-old former world number one Djokovic is in the evening prime time slot on Rod Laver Arena and will be hoping the heat has abated by then.

Djokovic has looked in imperious form in his two matches so far, yet to drop a set and striking a confident tone.

Djokovic, who has been scaling back the number of matches he plays to wring the most out of his ageing body, said: “I’m really happy I’m able to play this way, considering the lack of competitive matches for more than two months.

“It’s all been positive so far, positive signs.”

Before that, in what is likely to be the hottest part of the day, men’s champion Jannik Sinner of Italy plays another American in 85th-ranked Eliot Spizzirri on Rod Laver.

Second seed Sinner is bidding to win the Melbourne major for a third time in a row, emulating Djokovic.

Last up on centre court on Saturday is two-time Melbourne Park champion Osaka, who wowed the crowds earlier this week when she emerged for her opening match in a jellyfish-themed outfit.

On court she has not been so dazzling and faces home hope Maddison Inglis, a qualifier.

The winner goes up against either six-time Grand Slam champion Swiatek or 31st-seeded Russian Anna Kalinskaya.

Swiatek is aiming to complete the career Grand Slam of all four majors, with Australia the only one missing from her CV.

Other top seeds in action include number five Lorenzo Musetti of Italy, who clashes with Tomas Machac of the Czech Republic.

The winner goes up against either American ninth seed Taylor Fritz or the Swiss veteran Stan Wawrinka.

The 40-year-old Wawrinka must try and rouse himself on his Australian Open farewell after a gruelling five-set win in round two.