Billie Jean King Also Said Serena Couldn't Hang with Men

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."

She added, "I've never played anyone ranked 'there' nor do I have time. Respect me and my privacy as I'm trying to have a baby. Good day sir."

But the Serena vs. Men debate has been going on for a while -- and back in August, Billie Jean King told us pretty matter of factly that Serena would have zero chance against the best men on the planet.

BJK is a TITAN in women's sports -- most notably for pummeling Bobby Riggs (who was 55) in a co-ed tennis match dubbed the "Battle Of The Sexes" back in 1973.