Only Djokovic can do it! Federer too old, Nadal too prone on injuries even when at his best, no ESPECIALLY when at his best, because when he plays at his best it usually plays huge toll on his body, negatively affecting it (just look shat happened in the semis against Del Potro this year at USO!)...Djokovic is the only one of the group unaffected by these factors! (His elbow injury is a very rare exception!) and his arguably more complete/universal than Nadal! One of the pundits says (i believe it is McEnroe!) before this year's USO that wherever he plays he always has to change tactics and adjust, unlike Djokovic who can play the same on every grand slam and still keep winning! Its much harder to stay consistent, while adjusting each time, Federer had it back when he was much younger, but these days he barely ever play clay court matches let alone RG in particular! Djokovic can win way more than 6 more slams, i would even go as for as 11 or 12, he will be capable of pulling off a 3 slams seasons, like those back from 2011 and 2015 in every season he will compete until at least 2021 maybe even 2022 as well! Granted a lot of this depends of how much current youngsters will step up, but remember we had this kind of conversation when talking about the likes of Dimitrov and Raonic a few years ago and yet here we are! I think people give too much credit to current youngsters, when they expect them to make sufficient breakthrough to break the domination of an old army...in my opinion while both Nadal and Djokovic still playing its not going to happen...at leads at grand slams! It is way more likely they will retire undefeated or unbothered by the current gen of youngsters, than that they will start struggling against them at some point...just my two cents though! Djokovic and Nadal, especially former will be Steffi Grafs of male tour when all said and done! (Steffi was an active Roland Garros title holder when she retired and her last match was her defeat in Wimbledon final!...she didn't retire because she felt she can no longer compete, she retired because of marriage...i am expecting the same for Novak! When all said and done, the only factors that will keep him from keep winning granslams will be outside ones, not the competition...) It may sound like i am looking at a crystal ball right now at the moment, but it comes from my personal observation, made considered on the situation in male tour, that never chahnged ever since Gulbis, Gasquet and Monfils were all believed to overtake or at least challenge Roger's numbers! The only difference is that back than it was young gulbises, monfilses and gasquets and now it is young zverevs, tsitsipases and kyrgioses! They are not as special as people give them credit...