Teenage qualifier Learner Tien said it was "pretty crazy" after becoming the youngest man to reach the Australian Open fourth round since Rafael Nadal in 2005 when he overcame Frenchman Corentin Moutet on Saturday.

The American 19-year-old won 7-6 (12/10), 6-3, 6-3 in Melbourne with Moutet collapsing clutching his leg in the third set before gamely carrying on.

The victory extended Tien's remarkable tournament after he stunned last year's runner-up and fifth seed Daniil Medvedev in a five-set thriller in round two.

"Honestly it feels pretty crazy to be in the second week. Going through Qs (qualifying)," he said.

No other American man his age has gone so far in Melbourne since Pete Sampras in 1990, with Tien's reward a clash against Italian Lorenzo Sonego, who dispatched Hungary's Fabian Marozsan in four sets.

"This has exceeded my expectations, what I was hoping for coming into this week," added Tien.

"You go into every match believing you can win, but to be in the second week is amazing."

Tien is one of a trio of teens who set the Australian Open alight with wins over top-10 players, alongside Brazil's Joao Fonseca and Jakub Mensik, who have both been knocked out since.

In a battle of left-handers, Tien broke early for a 3-1 lead in the first set on the back of an unforced baseline error from Moutet. But the Frenchman levelled at 3-3.

Another exchange of breaks sent it to a dramatic tiebreak that ebbed and flowed before Tien clinched it 12/10, ending a 72-minute set.

The American broke twice in set two to seize control of the match before Moutet collapsed to the ground clutching his left leg after serving at 15-0 in the opening game of the third set.

He got back up and gamely attempted to continue, but was clearly in pain with his movement restricted and Tien took advantage to book a place in a maiden Grand Slam last 16.

"I didn't really see what was going on... I just saw him on the ground," Tien said.

"I wasn't sure what was going on with him honestly because there were some points where it seemed like he wasn't able to move or really use his legs that much at all.

"Other points he was running and scrapping."

mp/pst