Ailing defending champion Jannik Sinner admitted he was fortunate to survive a fourth-round battle with Holger Rune at the Australian Open.

Sinner looked in deep trouble after losing the second set and he took a long medical timeout early in the third as he battled to stay on level terms.

But Rune was also feeling the heat and a bizarre delay at the beginning of the fourth set when Sinner broke the net with a serve also played into the Italian’s hands.

He eventually completed a 6-3 3-6 6-3 6-2 victory, saying in his on-court interview: “It was for sure very, very tough.

“I knew in my mind that also he had some very long matches so I tried to stay there mentally. I just tried to stay connected with my service games and then in the return games see what happens.

“This morning was a very strange morning. I didn’t even warm up today, trying to go on court as fit as I could. I knew in my mind I would struggle today. Game wise, I played very well. Quality shots that gave me the strength to fight on.”

In his press conference, Sinner added of his struggles: “I was not feeling really well. I think we saw that today. I talked with the doctor already, before going on the court. I was a bit dizzy at times. I don’t want to go into details.”

The top seed also admitted the broken net was a major stroke of fortune, saying: “At this point, 20 minutes off court, trying to get back physically, putting some cold water on my head, it was very helpful. It was big, big luck to me today.”

It was hot and humid at Melbourne Park and Sinner began to look very lethargic during the latter stages of the second set.

He could be seen shaking at one changeover and Rune tried to pile on the pressure early in the third, with Sinner saving three break points, one with a volley into the open court after an incredible 37-shot rally.

At 3-2, Sinner called the trainer and doctor and had his oxygen levels taken before heading off court, with the break lasting just over 10 minutes.

On the resumption he managed to regain control, breaking the Rune serve in a long eighth game, with the Dane then taking his own timeout for soreness in his knee.

Sinner successfully served out the set and, at the start of the second game of the fourth, a serve from the 23-year-old broke the fixing connecting the middle of the net to the court.

It took 20 minutes, with the players heading off court, for the problem to be fixed, and Sinner made short work of the rest of the contest on the resumption.

The Italian has now won 18 matches in a row dating back to a final loss against Carlos Alcaraz in Beijing at the beginning of October.

Rune had no objections to Sinner taking the medical timeout but felt it did change the momentum of the match.

“I think obviously it’s perfectly fine that you check,” said Rune. “It was a very warm day today, even though there was not sun the whole time. It was very humid, so I was also feeling it.

“I think it took longer than I expected. So that was a bit brutal in the middle of the set. I just couldn’t move on. I had good momentum in this moment. So it was not the worst timing from his side.

“Especially having around 10-12 minutes break, just sitting in the heat for me cooking out there is a bit tough.

“They checked him on the court and the umpire said he needed more checks to make sure he was alright. Then he came back firing, so I don’t know what they did.”

Sinner was joined in the quarter-finals by countryman Lorenzo Sonego, who is through to this stage at a grand slam for the first time after ending the run of 19-year-old American Learner Tien with a 6-3 6-2 3-6 6-1 victory.