Andy Murray was so pissed that he couldn't take a piss during a marathon match at the Australian Open on Thursday night ... he unloaded on the chair ump, calling her refusal of a potty break a straight up "joke."
Murray and Thanasi Kokkinakis battled for over FIVE HOURS at the famed tennis tournament in Melbourne ... and just the before the match went to its fifth set, Murray asked to hit the toilet.
The problem? He had already been once previously during the contest -- so, the ump refused his request.