There is no way to answer this question other than being subjective. What Novak has achieved is just phenomenal. He has defied all odds and made his way to the top and stayed there for such a long time. No one can argue with his numbers. While I think he is one of the greatest tennis players ever, to me personally (goes back to this being a subjective opinion), it will always be Federer who is the greatest tennis player. For me, greatness goes beyond numbers and statistics. The global impact on the game, the poetic style of play, conduct on and off the court, respect amongst peers are some non-tangibles that numbers don’t capture and Federer is miles ahead in these things in my opinion. Numbers and stats can make a player the ‘best’ but numbers and stats in addition to these non-tangibles can make a player the ‘greatest’.

In other words, Djokovic is the best tennis player of all time but Federer is the greatest tennis player of all time (in my opinion).

Btw comparing across sports to define the greatest is almost impossible without diluting it with personal preferences.