MELBOURNE, Australia — First came the medical timeouts, one each for Jannik Sinner and Holger Rune with the temperature above 90 degrees Fahrenheit at the Australian Open. Then came the unusual sight of a 20-minute delay because the net at Rod Laver Arena detached from the court after being hit by a big Sinner serve.

In the end, Sinner put his physical struggles aside and emerged with the victory — as he keeps doing, no matter the site or the circumstances — and the defending champion moved into the quarterfinals at Melbourne Park on Monday by eliminating the 13th-seeded Rune 6-3, 3-6, 6-3, 6-2.

The No. 1-ranked Sinner occasionally tried to cool off by pressing a cold towel to his face or pouring water down the back of his neck. He was far better down the stretch, both after a 10-minute-plus delay in the third set when he went to the locker room for medical attention and after a 20-minute holdup in the fourth when the screw connecting the net to the blue playing surface came undone.

“I knew in my mind … I would struggle today,” Sinner said during his on-court interview, without saying what was wrong. “Me and the doctor, we talked a little bit. It helped me.”

He has won 18 consecutive tour-level matches, dating to late 2024. Last season, Sinner went 73-6 with eight titles, the first man with that many tournament championships in a single year since Andy Murray in 2016.

That haul included Sinner’s first two Grand Slam trophies, at the Australian Open in January and the US Open in September, the latter shortly after he was exonerated for testing positive for an anabolic steroid twice in March. His case is still unresolved, though, with a hearing scheduled for April in the World Anti-Doping Agency’s appeal of the ruling.

Rune, a 21-year-old from Denmark, was trying to get to the quarterfinals in Melbourne for the first time.

Sinner will face No. 8 Alex De Minaur of Australia or unseeded Alex Michelsen of the U.S. for a berth in the semifinals. A second Italian joined Sinner in the quarterfinals when 55th-ranked Lorenzo Sonego got that far at a major tournament for the first time by ending the run of American qualifier Learner Tien 6-2, 6-3, 3-6, 6-1. Sonego will now face No. 21 Ben Shelton of the U.S. or Gael Monfils of France.

The men’s quarterfinals Tuesday will be Novak Djokovic against Carlos Alcaraz, and Alexander Zverev against Tommy Paul.

The first signs of trouble for Sinner came with Rune serving at 3-all in the second set.

After lunging for a shot behind the baseline and stumbling slightly, Sinner clutched at his upper left leg and looked as though he might be bothered by some sort of discomfort, although it wasn’t entirely clear what was going on.

After Rune held there, Sinner walked slowly to the sideline for the ensuing changeover and was breathing heavily during the time between games. When play resumed, Rune earned his first break point of the match, and Sinner handed it over with a double-fault that made his deficit 5-3.

Rune closed out that set, and Sinner momentarily sat on his bench before walking slowly off the court and toward the locker room.

It was a muggy afternoon, and long, physical points left both players spent. After a pivotal, 37-stroke exchange in the third — claimed by Sinner with a cross-court swinging forehand volley passing winner after bringing Rune forward with a drop shot — each man leaned over with hands on his knees, gasping for air. One of Sinner’s coaches, Darren Cahill, stood in his courtside box and raised his left fist.

Then, during the changeover at 3-2 in the third set, Sinner asked the chair umpire to call for a trainer, and told a ball kid to bring him a bottle of something to drink from his team. Sinner’s pulse was checked, and then he trudged off with a towel draped around his beck and a bottle in each hand, accompanied by a doctor.

When action resumed, Rune was the one playing a bit recklessly and without an effective game plan, and he got broken to trail 5-3 — then immediately requested his own medical check, during which his right knee was massaged by a trainer. That might actually have helped Sinner.

“It was, for sure, very, very tough,” Sinner said. “I knew in my mind that he had some very long matches before this one, so I tried to stay there mentally.”

The Associated Press contributed to this report.