I have said since 2007 that Djoko was the only active player who could challenge Fed's records on hard. Now Fed has 9 slam titles on hard and 6 WTF titles. Djoko is already 27 and nowhere close to those stats but he's making top 5 on hc which is already a fantastic achievement.
Hard court titles overall: 1- Federer: 53
2- Agassi: 46
3- Connors: 43
4- Sampras: 36
5- Djoko: 34
(Those do not include carpet, which is not fair to guys like Lendl, McEnroe and Connors since lots of events were played on carpet instead of hard in their time. If you combine hc + carpet, the tally becomes quite different indeed and Fed would be nowhere near #1...)
The slam tally is also partial to recent times since USO was played on hard only from 1978 and AO from 1988:
1- Federer: 9
2- Sampras: 7
3- Agassi: 6
4- Djoko and Lendl: 5
(All of those players won both slams on hard)
YEC (was played on carpet before hc):
1- Federer: 6
2- Lendl and Sampras: 5
3- Nastase: 4 (3 of which on carpet)
4: Becker, McEnroe, Djoko: 3
For (current) masters, I included Paris indoor even though it was played on carpet in the 90s:
1- Agassi: 14 (+ 2 Paris indoor)
2- Fed: 15
3- Djoko: 14
4- Sampras: 8 (+ 2 Paris)
5- Murray: 9
(Note that Agassi and Fed are the only 2 players to have won all 6 masters on hard/carpet)
Winning % on hard (does not include carpet):
1- Connors: 82.7
2- Lendl and Djoko: 82.6
3- Federer and Laver: 82.5
4- McEnroe: 81.1
5- Sampras: 80.4
(only category where Djoko has passed Fed and of course it could change with aging)
I don't see a way to reasonably predict that Djoko will surpass Fed at this point (except most likely in master titles). It is not impossible but he's still far enough that one cannot say that it's likely either.
The main question for now is how close he will get over the next 5 years or so.