J.K. Rowling -- DESTROYS SERENA WILLIAMS HATER ... She's Beautiful, You Moron!

J.K. Rowling just went all AVADA KEDAVRA on some pathetic cyber troll -- EVISCERATING an "idiot" who talked smack about Serena Williams.

The "Harry Potter" creator was giving mad props to Williams on Twitter for winning Wimbeldon -- saying, "I love her. What an athlete, what a role model, what a woman!"

Enter some schmuck named Rob, who tweeted back at Rowling -- "Ironic then that main reason for her success is that she is built like a man."

That's when Rowling delivered the "drop the mic" moment -- posting a pic of Serena looking SMOKIN' HOT in a dress ... and adding the caption, "'she is built like a man'. Yeah, my husband looks just like this in a dress. You're an idiot."

Goodnight Rob.