Djokovic has been at the top of men's tennis since 2011, and this is demonstrated by the fact that he has won the season-ending championship for the fourth consecutive year. In this year's tournament he finally lost a match, but it was in the round-robin format so he was still able to qualify for the semifinals an d get revenge in the final. During this tournament Djokovic evened out his rivalry with both Rafael Nadal (23-apiece) and Federer (22-apiece). It seems more likely than not that the Serb will end his career with leads against both these champions since both Federer's and Nadal's peaks are definitely behind them but Djokovic's may still be yet to come.
Djokovic won 27 of 28 grand slam matches in 2015 and went 82-6 overall, winning 93.2% of all matches he played. This chart puts his remarkable 2015in the context of some of the best years of all time:
Of course, I still think Federer has the edge in any conversation about the Greatest of all Time because he not only has 17 major titles (the most ever) but also has been at or near the top of men's tennis for well over a decade (2003). Djokovic won his first major title in 2008 so he has until about 2018 to see how many more majors he can get, but I would not be surprised if it was in the 14-16 range. He currently is 10-8 in major finals (but he has won 3 of 4 he played in 2015, including 2 consecutive Wimbledon finals against Federer) while Federer is 17-10 and Nadal is 14-6. Ultimately, it will be his final record in these historic matches that will determine his place in the pantheon of tennis greats.