Sinner defies dizzy spells to reach Melbourne last eight
The world number one again dropped a set before coming home 6-3, 3-6, 6-3, 6-2 against Danish 13th seed Holger Rune on Rod Laver Arena with both men needing medical attention.
Sinner struggled in the muggy conditions, his hand visibly shaking and his heart rate taken in the third set before he left court for a medical timeout.
Rune needed attention to a sore right knee in the same set.
They then had a bizarre 20-minute delay when Sinner demolished the metal brace that secures the net to the floor with a thunderous serve early in set four.
"I was not feeling really well. You know, I think we saw that today I was struggling physically," said Sinner, 23, who is bidding to become the first Italian man to win three Grand Slam crowns.
He refused to say exactly what was wrong with him, only that he was "not there health-wise" and had been "a bit dizzy at times".
"I don't want to go into details. I think it was, you know, then also with the pressure and everything, it was not easy."
He admitted that the time off court in air-conditioning while officials repaired the net helped him refresh.
"I was lucky today that... 20 minutes off court, you know, trying to get back physically, putting some cold water in my head, it was very helpful. It was big, big luck to me today."
The win put him into a clash against Australia's big hope, eighth seed Alex de Minaur, or young American Alex Michelsen.
The Italian is bidding to defend a Grand Slam title for the first time after winning the first of his two majors in Melbourne last year, beating Daniil Medvedev in the final.
Labouring
As temperatures rose above 30 Celsius (86 Fahrenheit) for a second successive day, Sinner stamped his authority on the match by breaking to love, with Rune hitting four consecutive errors.
Sinner's serve was iron-clad and there was no sniff for the Dane, who went a set down in 33 minutes.
But a Sinner double-fault in the eighth game of set two handed Rune a break, allowing him to hold and level the match as he began to find his rhythm.
It was the second set Sinner had dropped in the tournament, having not done so in 13 previous matches.
Sinner clung on to hold in an epic game three in the third set, which included a remarkable 37-shot rally that the Italian won to save break point.
He gave Rune another break point at 2-2 with a double fault, but again saved, yet he was labouring.
At the changeover, a doctor checked him and he went off for a medical timeout.
Sinner returned and slogged through a testing 14-point eighth game to break for 5-3 before sealing the set.
After the delay while the net was fixed, he produced a top-notch forehand winner to break for 2-1 in the fourth set and there was no way back for a tiring Rune.