Billie Jean King

It ain't Martina. It's not Steffi ... Andre, Pete or McEnroe -- the greatest player to ever step on the tennis court is SERENA WILLIAMS ... so says Billie Jean King. In the past, Billie has raved about Serena's potential ... saying she has what it takes to be #1. Fast forward to Wednesday ... and…

Billie Jean King is calling BS on reports the mafia rigged her famous Battle of the Sexes tennis match with Bobby Riggs ... telling TMZ, "I just beat his butt and that's it." The allegations surfaced in an ESPN investigation last month -- a country club worker claims he witnessed a secret meeting…