I think the real question is why is Murray even in the conversation with the other 3? It's clearly been a BIG 3 this generation, not a BIG 4. Murray isn't on their level. Wawrinka has just as many slams. He and Murray are 2nd tier.
As for the OP, Novak is incredible, but a lot of this is timing. As others have said, Nadal and Federer are on the way out, and had each other to contend with, with an almost complete overlap in their primes. Federer would have at least a couple Calendar slams and probably 20+ if Nadal hadn't been around, and Nadal would undoubtedly have more slams too if not for Federer. Nadal is unlucky because as soon as he cracked Federer's dominance, Djokovic ascended, so he had constant overlap with two incredibly strong rivals during his peak. Novak is possibly (likely) going to exceed both Fed and Rafa in slams, weeks at #1, etc., but a fairly large part of that is the luck of timing with both of them being past their prime and nobody else stepping up yet. Novak's getting close to 30 and that WILL affect his game, but unless some of the younger players start having some belief, he'll still get some freebies when he's over 30 just because there's nobody to challenge him.
All this being said, I have insane respect for Djokovic, Nadal, and Federer. It's truly been a legendary generation. Probably the greatest ever, in my opinion. Will be interesting to see which guys step up to take their place. And cool to have Serena making history on the women's side--though I realize that's a whole other discussion in and of itself.
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