Serena Williams' husband is clapping back HARD at a billionaire who criticized the tennis legend ... with Alexis Ohanian calling the guy a straight-up "racist/sexist clown."
Here's the backstory ... Ion Tiriac -- the super-rich ex-tennis star who's the director of the Madrid Open -- went on a Romanian television show recently and jabbed at Serena.
"At this age and the weight she is now, she does not move as easily as he did 15 years ago," Tiriac said, according to ProSport in Romania.