Daniil Medvedev reached his boiling point during his Australian Open match on Monday ... getting so pissed at a spectator, he told them to "f*** off."
The altercation went down as the Russian star faced off against Marcos Giron in the opening round of the tourney ... when he was up 5-2 over the American after winning the first two sets.
But before Medvedev could handle business and finish Giron off, he confronted a fan in the stands for seemingly heckling him throughout the match.