Regarding the comment talking about the "new level" in tennis with no tactical room to be exploited, beyond which further optimization seems difficult...wow. Talk about swallowing the McEnroe-generated new-age marketing BS hook, line and sinker.
These guys are not products of the "evolution of tennis" who have went off with Tibetan monks and meditated to new tennis highs, successively higher than one another. They are exceptionally rare talents. Federer is a natural talent on all surfaces and, despite the canard spewed around here, worked extremely hard throughout his career to improve weaknesses. Nadal was a natural prodigy on clay, but knew he needed to work on his game immensely to make inroads anywhere else. Djokovic was very good on all surfaces but is quite obviously naturally mentally weak, and had to work on that, as well as his poor fitness and some of his technique, to have any chance.
Yes, because Federer was so good, he set the bar high for Nadal and Djokovic. However, they both had the natural talent to get to where they got (which still wasn't as natural or great as Federer).
But this wasn't some "evolution of tennis", it was guys realizing that to compete with Federer they needed to really push their limits. You also see this with Wawrinka.
However, the same thing could be said about Lendl in the 1980s.
These aren't, however, giants "leaps of evolution" in the game. It's more like a 6-sigma event: Three tier 1 all-time greats successively in somewhat overlapping eras. However, none of these guys are immune to the laws of nature, and to talk as if any of them are playing better now than in their physical prime is the height of lunacy. Djokovic isn't as good as he was in 2011, it's just that few can beat him (e.g., Wawrinka did in the FO, but Federer who stopped him in 2011 is now older and inconsistent).
Djokovic will not levitate where he is now for years to come unchanged before deciding to call it quites, anymore than Federer could have. Djokovic fans seem to forget that no one would have predicted Djokovic's 2011 after his early career mental weakness and poor fitness...and remember, it is an incredibly strict regimen that elevated him from that, and his fitness could easily fall back down if he suffers any lapses; his mental strength was also suspect again in 2013 and parts of 2014.
So just as Djokovic emerged suddenly out of mental fragility and poor fitness to greatness, the same too could happen with say Krygios, Cilic, or even Wawrinka for a year or two.
Then we'd hear more hogwash about the "evolution of the game" because people would parrot similar hogwash that Djokovic was playing "better" at say 34 than at 28.