World No. 1 Jannik Sinner overcame physical discomfort and a battling effort from gutsy 13th seed Holger Rune on Monday to reach the Australian Open quarterfinals.

After losing the second set, Sinner's title defense appeared to be hanging by a thread when he required medical treatment in the third set.

Sinner looked exhausted and was visibly shaking amid warm conditions on Rod Laver Arena before he regrouped 6-3, 3-6, 6-3, 6-2 in a see-sawing match lasting three hours and 13 minutes.

The two-time Grand Slam champion will next play Australian Alex de Minaur or 20-year-old Alex Michelsen in the last eight.

"It was very, very tough. I knew he had some tough matches and I tried to stay connected with my serving and returning game," Sinner said.

Noted for his aggressive return and defensive mastery, the 21-year-old Dane is considered one of the best young players in men's tennis.

Rune was aiming to make the quarterfinals in Melbourne for the first time. But he faced a mountain against Sinner after a grueling opening three matches, starting with a five-set epic against China's Zhang Zhizhen.

Sinner made a ruthless start, attacking Rune's serve and breaking to love before slamming consecutive aces to lead 3-0.

Rune slowly found his rhythm and held his service games relatively comfortably, but Sinner was untroubled on serve as he maintained his break advantage.

The first set ended tamely for Rune when he slammed a backhand return on a short second serve into the net.

Sinner appeared to be dictating terms and tried to crush Rune's spirit, but was unable to convert a break point in the fifth game.

A revived Rune summoned extra energy and hustled around the court, forcing Sinner into uncharacteristic errors.

Rune capitalized on an untidy service game from Sinner and broke after being handed a double fault before calmly holding serve to level the match.

Sinner was under a barrage from Rune and forced to dig deep to save four break points in the third game, winning one with a forehand winner after both players traded breathtaking strokes in a remarkable rally.

Sinner saved another break point in his next service game before requiring a medical timeout. He looked distressed and briefly spent time off the court, but returned refreshed and broke to take a 5-3 lead.

Rune received treatment for an apparent cramp as a waiting Sinner ate a banana before clinching the third set with an ace.

Both players had respite early in the fourth set after a Sinner serve broke the net leading to a 15-minute delay. Sinner settled quicker on resumption, breaking twice, and he sprinted towards the finish line.