The big-serving American 21st seed will face either defending champion Jannik Sinner or Australia's Alex de Minaur for a place in the final at Melbourne Park.

In a battle on Rod Laver Arena dominated by serve, Shelton had the edge over his unseeded Italian opponent for the first two sets.

The world number 55 refused to lie down however and took the third, before Shelton reasserted himself to win an attritional fourth set on a tiebreak.

"I'm relieved right now," said the 22-year-old Shelton, who unleashed the joint-fastest serve of the tournament, clocked at 232kph (144mph).

"Shout-out to Lorenzo because that was some ridiculous tennis," added Shelton, who reached only his second Grand Slam semi-final.

His first came at the 2023 US Open, where he was beaten by eventual champion Novak Djokovic.

"I'm just really happy to be through, getting my first win here on Rod Laver," added the left-hander, who is making his third Australian Open appearance.

Both players benefited from a quarter of the draw that was left wide open -- after the early exits of top-10 seeds Taylor Fritz, Daniil Medvedev and Andrey Rublev -- to reach the last eight.

All four sets were a tight affair and Shelton saved three break points to level at 3-3 in the first, finally manufacturing a break when the Italian went long in the ninth game.

Shelton sealed the opener with his monstrous ace.

Sonego then came up with one of the shots of the tournament to save a break point in the next game.

The Italian dived full-length for a drop volley that spun so wickedly it bounced back over the net before Shelton could retrieve.

It brought a huge smile and congratulatory fist bump from the American as Sonego milked the ovation from a packed Rod Laver Arena.

Shelton extended his lead to two sets courtesy of a rocket forehand winner, but Sonego wasn't finished and clawed back a set with a late break in the third.

Shelton finally got over the line via a tiebreak to secure his last-four berth in 3hr 50min.

Sonego, 29, had ended the runs of two of the hottest young properties in tennis and ousted a former champion on his way to his first quarter-final in a Slam.

He knocked out the 2014 Australian Open winner Stan Wawrinka in round one, then beat NextGen champion Joao Fonseca, 18, in a five-set epic.

In the fourth round he overcame 19-year-old Learner Tien, the conqueror of three-time finalist Medvedev.