Venus & Serena Williams -- We Can SING Like Madonna [Video]

Serena Williams took on Madonna this weekend ... not on the court (because she would smash Madge's face in) .... but at a Karaoke bar in Milan, where she belted out "Like a Prayer" ... like a CHAMP!

Serena and her sister Venus were in the country for an exhibition tennis match -- but Saturday night, the two partied their well-endowed asses off ... singing and dancing like maniacs.

The next day ... Venus suffered a wardrobe malfunction during a match with an Italian player ... but managed to cover up before anything got loose.