Melbourne, Jan 22 (IANS): Stefanos Tsitsipas passed a real test of his title-winning credentials on Sunday at the Australian Open, where the third seed held firm in the face of a Jannik Sinner onslaught to clinch a thrilling 6-4, 6-4, 3-6, 4-6, 6-3 fourth-round victory and reach the quarterfinals, here.

Having marched through his opening three matches in Melbourne without dropping a set, the Greek was presented with the biggest test of his campaign by 21-year-old Sinner.

For the first two sets, Tsitsipas stayed on track to repeat his straight-sets quarterfinal win against the Italian in Melbourne a year ago, but his hopes of a similarly one-sided result were dashed by a remarkable change in level from his opponent.

An increasingly confident Sinner dominated the baseline exchanges in the third and fourth sets to carry the momentum into a decider on Rod Laver Arena. Yet Tsitsipas retained his composure, dialled in behind his serve and huge forehand, and clinched a decisive break in the sixth game of the fifth set en route to a gripping four-hour win.

"It was a long match, guys, I feel like I spent an entire century on this court playing tennis," said Tsitsipas to the crowd.

"It felt so long. What a great night… I'm really excited to be sharing moments like this on the court, especially in Australia. I'm trying to do my best out here, it's not easy. I had an unbelievable opponent on the other side of the court today, playing incredible tennis in the third and fourth sets," he added.

The victory extended Tsitsipas' ATP head-to-head series lead against Sinner to 5-1. The Greek, who will rise to No. 1 in the ATP Rankings for the first time should he lift the trophy in Melbourne, will next take on Jiri Lehecka as he bids for a third straight Australian Open semifinal appearance.