I'm not sure he should be compared to them.
Djokovic is currently Tier 2, and I am becoming increasingly confident that he will end up in Tier 1.
But Fedal are different, and here's how I see the trio:
I think that Federer is at the point where at his best, his level actually becomes difficult to quantify in any meaningful way.
Essentially, he essentially maxes out how well tennis can be played within human physical limits, and in these moments watching Roger Federer play tennis is like watching the hand of God move a Wilson PS 90.
Nadal himself talks about this in his book, where he says that a large part of playing Federer in his prime was to simply weather the storm and wait for his level to drop so you could get back into the match. The guy's given Nadal 6-1 sets at RG before.
But Nadal is the master of handling Federer, something which almost seems as if it was intended by the divine, to keep another divine force in check and prevent it from making a complete mockery of the sport.
As such, Nadal himself must be almost equally great to be capable of this feat, so both are, in essence above the level at which their tennis can be quantified.
Rafa has something which Djokovic and even Federer do not, and that is an unbreakable will to win, which makes him the most reliable winner ever in my view, and possibly the best player, even if not the greatest.
Djokovic is an ATG no doubt, and will probably become Tier 1 too, but comparing him to a duo who, at their best, barely operate within the bounds of the possible is more than a bit unfair.
Fedal is the gold, nay, platinum standard of tennis to which all aspire.
Djokovic, I think, may have come the closest, and one thing I do really admire about him is that he is not content with this state of affairs.
He wants to join them, and become an Immortal.
And I wish him all the luck in the world in doing so.