Well there's the obvious cut-off of 10 Slams, but that's somewhat arbitrary. But a true "Mt. Rushmore" of tennis would have to imply players that have dominated their era, that is have been considered the best player at some point and for several years.
That 'tier one' business was probably me. When looking at the greatest players, I think the following are a step above the pack and all have some degree of GOAT cred: Bill Tilden, Pancho Gonzales, Ken Rosewall, Rod Laver, Bjorn Borg, Pete Sampras, Roger Federer, and Rafael Nadal.
That's eight players, so twice Rushmore. It would be hard to reduce it further. But for Novak to get into that group I think he needs 3+ more Slams and more titles in general. It is certainly possible. Right now he's in the mix with Connors, Lendl, McEnroe, and Agassi as "tier two" players.
While I wouldn't want to narrow the above list to four, an Open Era foursome would be a bit easier and that would be Borg, Sampras, Federer, and Nadal. Novak has a ways to go to pass Borg, but should surpass Lendl, Connors, Agassi, and McEnroe before he's through.
