Serena Williams Turns Rapper

Tennis superstar Serena Williams doubles as a fashion designer, spokeswoman, and part owner of the Miami Dolphins -- but now she's adding RAPPER to her resumé.

We're told Serena hit the studio last week to lay down a few tracks ... and is working with a record label owned by Minnesota Vikings star Bryant McKinnie.

During the session, we're told Serena did some work with renowned hip-hip producer DJ Clue ... and she wasn't half-bad.  Is there anything she isn't good at?