Daniil Medvedev Blows Up On Umpire At Australian Open, 'Are You Stupid?'

Tennis star Daniil Medvedev was straight-up PISSED with an umpire during an Australian Open match ... completely berating him and calling him a "small cat."

Medvedev was battling his rival, Stefanos Tsitsipas, for a shot at the final round of the tourney ... when he was issued a code violation by umpire Jaume Campistol.

The penalty came after the ump thought Daniil gave him an inappropriate gesture out of frustration ... and that's when the feisty Russian tennis sensation absolutely lost it.

Medvedev was clearly furious at the ump over the penalty ... as well as for not penalizing his opponent, claiming his father was coaching him during the match.

"Bro, are you mad?" Medvedev yelled to the ump. "For what? And his father can talk every point? Bro, are you stupid? His father can talk every point!"

"Oh my God, you are so bad, man," he continued. "How can you be so bad in the semifinal of a Grand Slam. Look at me! I’m talking to you!"

The umpire, to his credit, maintained his composure even as Medvedev continued to go off. "If you don’t [give Tsitsipas a warning], you are ... a small cat," Medvedev yelled.

Tsitsipas -- who lost to DM in four sets -- did admit that his father was talking during the match, but offered an explanation.

"My father, his medicine is to talk, and you can’t stop it,” Tsitsipas said. "I’m pretty sure I’m going to keep receiving coaching violations, even though I will never listen to any single thing he says."