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."
Sinner makes last 16
A misfiring Jannik Sinner dropped a rare set before taming American underdog Eliot Spizzirri in a drama-filled clash in wilting heat to make the last 16.
The two-time defending champion was broken three times in the first set and six times during the match as he laboured to a 4-6, 6-3, 6-4,6-4 victory on Rod Laver Arena over a gruelling 3hr 45min.
His reward is a clash next will fellow Italian Luciano Darderi, the 22nd seed.
With temperatures climbing to a forecast 40C, it was not the demolition job many predicted against a player ranked 85 with Sinner struggling to acclimatise in the brutal conditions.
Draping himself in ice towels during the changeovers, he suffered a fright in the third set with severe cramp that needed treatment.
Hobbling, he was only saved when play was halted due to the extreme heat and the stadium roof closed.
It was the first time Sinner had dropped a set in 12 matches dating back to Vienna last year against Alexander Zverev.
"I struggled physically a bit today as you saw. I got lucky with the heat rule, they closed the roof," he said.
"As the time passed, I felt better and better, and very happy about this performance.
"Looking back, in every big tournament I've had some really tough matches. So hopefully this can give me some positives for the next round."
Sinner is striving to win three Melbourne titles in a row and join Novak Djokovic as the only men to do so in the Open era (since 1968).
He came out firing, holding serve then securing an immediate break before inexplicably letting his guard down to be broken back to love.
The imperious Italian got in his groove and broke again with a searing crosscourt forehand winner for 4-2, but once again was broken to love.
Sinner struggled in the heat, broken for a third time to slide 5-4 behind with a composed Spizzirri serving out the set in 43 minutes.
Incredibly, Spizzirri broke again for 1-0 in the second set, leaving Sinner shaking his head.
The world number two slowly began to figure out the American and broke back and held for 2-1. With momentum back in his favour he made no mistakes to take the set.
But the energy-sapping weather took its toll and Sinner was left limping after a 20-shot rally in the third set.
He was in real trouble and slumped 3-1 behind before the extreme heat rule was applied and he left the court while the roof was closing.
When they resumed, he broke straight away, looking more comfortable out of the sun's glare, with another break leaving him serving out for the set.
Unfazed, Spizzirri kept applying pressure and the pair exchanged breaks in the fourth set before Sinner finally seized control.
