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 BJK tells TMZ Sports she's convinced -- Serena's the best to ever do it.
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