Sly Stone -- Lawyer Suffers Supreme KO From Fake Diana Ross

Sly Stone's lawyer got the Supreme diva treatment outside court -- taking a punch to the face from a woman claiming to be Diana Ross.

Law enforcement sources tell TMZ ... minutes before Stone and his lawyer entered an L.A. courtroom Friday -- a woman walked up, announced she was THE Diana, then socked the attorney so hard his face was gashed.

Witnesses say the woman accused the lawyer of trying to get Sly locked up ... and popped him when he talked back. We're told cops pounced, and arrested her for misdemeanor battery.