Billie Jean King Says, 'DON'T QUIT ON HILLARY, It Ain't Over Yet!'

It ain't over 'til it's over -- so says Billie Jean King who says she's not ready to throw in the towel on Hillary Clinton ... yet.

The tennis legend is at the Clinton party at the Javits Center in NYC -- where she told us she remains hopeful HC can pull this thing off -- despite the fact Donald seems to be kicking her ass.