IMO, the Greated Tennis Player of All Time is Gardner Malloy. He is the only player in the history of the game who has maintained world top-ten status within his age group over most of a seven-decade adult playing career. (If he competed in the juniors, one might be able to add an eighth decade.) Heck, lots of other top players didn't even _live_ for seven decades, much less compete in tournaments as an adult for that long. Bobby Riggs would be a distant second place.