It all went down at the Melbourne Summer Set ... when a bird decided to do its business right on the (ironically enough) #2 seed's hat during the second set against Maxime Cressy.

Opelka -- the #26 player in the world -- was immediately ticked off by becoming a human toilet ... walking over to the chair umpire in hopes of switching out his hat.

"Sorry, I just got s*** on," Opelka told the umpire as his sweaty locks draped over his eyes. "I can't see, a bird s*** on me. What do you want me to do?"

What unfolded over the next few minutes was nothing short of a movie -- Opelka tried wearing a different hat, but was told the logo on it was too big.

Next, the judge suggested wearing his coach's hat -- but Opelka refused to wear a different brand.

Even a ball boy tried to come to the rescue -- offering his Australian Open lid to the player, but it wouldn't fit his head.

Eventually, a match official brought out a tournament-approved hat for him to wear and asked if he wanted it swapped with a different hat in his locker room ... which Opelka accepted.