but how can you be the greatest, if there's someone better than you?
Any "greatest of all time" moniker as to be implied along a stretch of time.
over the stretch of his career (not yet finished) federer is one of only 6 players in history to have won the (career) grand Slam, and the one with most titles (14, with Sampras, who never won Roland Garros).
And he's likely not done yet.
so, how is he
not the greatest?
and right NOW, he actually
is the best, as there is no one better then him, since he just won the most recent important tournament.
Most would agree that Nadal is obviously the best player of the last year, but he just lost to soderling, so how can he be the best if there is someone better then him

? If you don't allow for stretches, the 'best' is whoever won the last tournament.
Jokes aside, i think currently a healthy Nadal is superior to Federer, but by a very narrow margin, and things could change come Wimbledon and the US Opens.
the problem with Rafa is that his game is so much based on physical prowess and athleticism, that the moment that is not top (like now), he cannot fully compensate with technique. Such moments are bound to happen in anyone's career, so his is likely to be shorter than federer's (at least at the very top level) and with more frequent 'pauses'. I think that is what was meant with the "it's fair game' comment, that if one's game is so based on power/athleticism, drops are to be expected.
Serena is the same kind of player. Easily the best when in form, but cannot maintain it for extended periods.
that said, if Federer and Nadal produce another similar Wimbledon final, I don't care who wins
