Sly Stone Lawyer Suffers Supreme KO From Fake Diana Ross

3/22/2015 12:50 AM PDT

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

EXCLUSIVE

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.

Sly was going to court for a hearing on his $5 million royalties case -- but it was postponed after the attack.