To be the greatest, you need to have someone else who is greater.

It's a matter of comparision. If everyone is great, then you are only "greater" than the rest. But only if there are a handful "greater", can you aspire to be the "greatest".

That was the caveat Roger Federer was trapped in. During the years that he was the undisputed king of the game, Federer was exactly that, the "undisputed" king.

Among all the good players, he was "better".

Then came Rafael, "better" than the good players, and thus started the quest to become the "greatest".

Notwithstanding st what level of ability one consistently performs, it is judged superlative only when there is another entity to compare it with.