World No.1 Jannik Sinner’s fitness has been placed under a cloud of uncertainty as rumours he is battling a serious illness swirl around Melbourne Park.

The 23-year-old sparked concern during his fourth round win over Holger Rune as he was seen trembling and struggling to breathe between points.

Having required a medical timeout for “dizzy spells” during the match, there has since been “unconfirmed reports” that Sinner may be recovering from pneumonia.

The rumour adds a new layer to Sinner’s upcoming quarter final clash, in which he will face Australian hope Alex de Minaur.

De Minaur has a horrible record against the top-ranked player, yet to taste success in nine meetings, but his beefed up game and strong form puts him in the strongest position yet to test the rangy Italian.

Former Australian player John Millman revealed he had not seen Sinner return to the practice courts since his victory over Rune.

“One thing I’ll say – and I’m no expert and maybe cameras picked up something else – but I didn’t see him at the courts yesterday,” Millman said on Nine’s broadcast of the Australian Open on Wednesday morning.

“It was actually quite distressing (to watch against Rune)… when you push your body beyond the limits you need more than a day to recover.

“I saw Alex, he was hitting. I didn’t see Jannik Sinner.”

But in another twist to the Sinner mystery, Lleyton Hewitt later revealed he was “pretty sure” Sinner was working out on one of the indoor practice courts.

“It was hot conditions, he and Alex spent time indoors which I think is smart coming into a big quarterfinal and not wasting any energy,” Hewitt said.

Despite the speculation around Sinner’s health, Hewitt admitted that de Minaur is expecting the two-time major title winner to come out firing.

“You’ve got to expect him to come out and go at 100 per cent right from the start,” he said.

“He was just having an average day a couple of days ago where he wasn’t feeling his best coming into the match but the big thing for Alex is he’s done the work.

“He knows he can last five sets and that’s what he has been training so hard for for so long but he’s up against it tonight.”