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.
You ball hard no tennis rackets...