Serena Williams -- Check My New Rap Song!

Forget tennis, Serena Williams got a new sport ... the rap game -- and she's making her debut with a brand new song that's actually pretty catchy.

TMZ broke the story ... Serena laid down some tracks last year at a Florida record studio called B Major Music Group -- owned by Baltimore Ravens offensive tackle Bryant McKinnie, who's good friends with the Williams sisters.

Now, one of the tracks has finally surfaced -- and it ain't half bad.

The lyrics are a little cliché -- shocker, another song about "balling hard" -- but her flow is surprisingly not annoying. She even gives her sister Venus a little shoutout.

Take a listen. Or don't. It's Friday. Do what you want.