Fifth seed Lorenzo Musetti battled stifling conditions to set up a last-eight showdown with Novak Djokovic and six-time major champion Iga Swiatek progressed in rampant fashion.

Two-time defending champion Sinner raced to a 6-1, 6-3, 7-6 (7/2) victory over fellow Italian and close friend Luciano Darderi.

The second seed plays the United States' all-action eighth seed Ben Shelton after he beat Norway's 12th-seeded Casper Ruud in four sets.

Djokovic and Sinner are on course for a blockbuster last-four showdown.

Sinner told Margaret Court Arena, where temperatures hovered around 30C in the early evening: "It was difficult to put the match away, happy I closed it in three sets."

Sinner sent down a bumper 19 aces in the match, and said his hard work had paid off.

"Still room to improve but happy how I have come back in the new season," he added.

In another clash between compatriots and friends, Jessica Pegula defeated title-holder Keys, 6-3, 6-4.

The Americans host a tennis podcast together, "The Player's Box", and had even been planning to record an episode on the eve of their match.

All that was put aside as a clinical Pegula dominated to set up a last-eight encounter with another American in fourth seed Amanda Anisimova.

Anisimova, beaten finalist at last year's US Open and Wimbledon, cooled herself off with bags of ice before taming China's Wang Xinyu 7-6 (7/4), 6-4.

Pegula, the sixth seed, is chasing an elusive Grand Slam title aged 31, her best performance to date reaching the US Open final in 2024.

An erratic Keys made 27 unforced errors to Pegula's 17 and fired down six double faults as her title defence folded.

The forfeit, as a part of a jokey bet with Pegula, will be a slice of apple pie with melted cheddar cheese on top.

"A bet is a bet, so I'll do it. I hope it's less gross than I think it's going to be, but we will find out," said Keys, who described herself as proud despite defeat.

Fifth seed Elena Rybakina dismantled 21st seed Elise Mertens of Belgium 6-1, 6-3 and will play second seed Swiatek.

The Pole demolished home qualifier Maddison Inglis 6-0, 6-3 as she closes on a first Australian crown, having won all three of the other majors.

Inglis, ranked 168, was in the biggest match of her life after two-time Melbourne champion Naomi Osaka pulled out injured from their third-round meeting.

Just being in the fourth round earned Inglis a life-changing Aus$480,000 (US$330,000) -- she plans to buy a toaster and a kettle with the windfall.

- Melbourne sizzles -

After brutal weather caused the suspension of matches on Saturday, temperatures rose sharply again.

Longer matches on Monday had 10-minute breaks in certain circumstances as part of measures to protect player health.

Temperatures are forecast to hit a blistering 45C on Tuesday, meaning matches are likely to be played under roofs on the three courts that have them.

In the last match of the day on Rod Laver Arena, Shelton defeated father-to-be Ruud 3-6, 6-4, 6-3, 6-4, letting out an almighty roar in victory.

Melbourne Park king Djokovic was supposed to face the 20-year-old Jakub Mensik in the night match.

But the Czech player pulled out injured on Sunday, sending 10-time champion Djokovic into the last eight without hitting a ball.

The 38-year-old Serb will play Musetti after the Italian defeated American ninth seed Taylor Fritz 6-2, 7-5, 6-4.

Djokovic has won nine of their 10 previous meetings, but Musetti said: "I feel ready to try to push him to his maximum."