Teenage rising star Joao Fonseca was bundled out in the Australian Open second round on Thursday by wily Italian Lorenzo Sonego in a five-set thriller.

The highly rated Brazilian, 18, burst on the scene by stunning ninth seed Andrey Rublev in round one and looked on track for another upset when he won the first set in Melbourne.

But in a seesawing battle, the 55th-ranked Sonego clawed through 6-7 (6/8), 6-3, 6-1, 3-6, 6-3 after 3hrs 37mins of high-quality tennis.

The loss ended a 15-match win streak for Fonseca, who was tipped for the top by Carlos Alcaraz and Novak Djokovic this week.

Sonego's reward in a clash against Hungary's Fabian Marozsan, who took down 17th seed Frances Tiafoe, also in five sets.

Little separated Fonseca and Sonego in a first set which went to a tiebreaker, where the Brazilian whipped up the crowd and got over the line.

But he was brought back to earth when a series of unforced errors handed Sonego a break for 2-1 in the second set.

The relentless Italian kept pounding away to earn another break for 5-3, with his big serve giving Fonseca little leverage.

The Brazilian's game began to lose its lustre and he was broken to love, missing a simple net volley, to fall 3-1 behind in the third set.

With his error count rising there seemed to be no way back.

But the teenager was not done and a Sonego double fault handed him a break for 2-0 in the fourth set, holding on to the send the match to a decider.

With the crowd firmly behind him, Fonseca had a break point at 3-3 but failed to covert.

It proved costly, with Sonego breaking in the next game and serving out for the match.

Fonseca last month became the second-youngest champion of the NextGen ATP tournament since current world number one Jannik Sinner claimed the title, also aged 18, five years ago.

He then won the ATP Challenger Tour title in Canberra before powering through Melbourne Park qualifying.

(AFP)

Brazil's Joao Fonseca reacts during his first round match against Russia's Andrey Rublev. (Reuters)

Teen star Fonseca out of Australian Open in five-set thriller