American firebrand Tennys Sandgren has blown up deluxe at the chair umpire early in his victory over Italy's Fabio Fognini.

Sandgren won the first set 7-6 before launching a tirade against the umpire, accusing him of preferential treatment toward Fognini.

Sandgren was furious after Fognini was allowed a toilet break at the end of the first set.

"He gets his own rules because you're afraid to step on his toes," Sandgren said.

"If you're wondering why players think you guys give people different treatment, this is one of the reasons why.

"I think he gets different treatment."

Fognini was given a point penalty for taking his shirt off and was booed by the Melbourne Arena crowd after the opening game of the second set.

He also called for treatment on his battered hand.

However, the American proved too hot for the Italian who was nursing a battered hand, winning 7-6 (7/5), 7-5, 6-7 (2/7), 6-4 to set up a last-eight clash against either Roger Federer or Hungarian Marton Fucsovics.

"That was fun. Playing him was just a war," said Sandgren, ranked 100.

"He is so good. You can never count him out at any stage. I had expectations of a fight and that's what we had." Sandgren, a devout Christian, achieved his best Grand Slam result at Melbourne Park by reaching the last eight in 2018, but it was overshadowed by a row over his political views and links to right-wing activists.

He has put the controversy behind him and shown battling qualities in his run to the quarter-finals this year, having also knocked out eighth seed Matteo Berrettini in the second round.