John McEnroe isn't the only one who thinks Serena Williams would get crushed on the men's circuit -- Billie Jean King told us the exact same thing last year ... and backed it up with science.
Of course, McEnroe's been getting killed over an interview this week in which he praised Serena as the best female player in the world but noted, "If she played the men's circuit she'd be like 700 in the world."
Williams shot back on Twitter saying, "Dear John, I adore and respect you but please please keep me out of your statements that are not factually based.