3 reasons why Novak Djokovic can surpass Roger Federer to become the leading Grand Slam winner of all time 

2) Style of Tennis

When talking about the style of tennis, I am certainly not advocating as to whose style is better but the crucial factor in Djokovic's style of tennis is the sustainability and the way it helps his body.

Amongst the Big Four, he's suffered the least amount of injuries and his superb backhand strength as well as his ability to master groundstrokes has played a terrific part in harbouring him a considerable amount of success because he has remained less injury-prone.

Unlike Rafael Nadal, who tends to play an aggressive baseline tennis or Stanislas Wawrinka, who has a similar pattern but has fitness issues, Djokovic, apart from a major elbow injury, has sustained himself well enough. At this age where he's more prone to injuries as body is getting older and recovery period gets extended, the Serb still has the lowest risk when it comes down to his playing style and how it will sustain him to progress further in the long run.

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3) The decline of 'FeDal' and the lacking of a new dominant face

As pointed out earlier in the article, both Roger Federer and Rafael Nadal are reaching the very end of their careers with both players having addressed questions based on retirement. Where Federer looks to be the earliest amongst the Big Three to call it quits, Nadal's injury record is very unconvincing and the Spaniard regularly has to battle fitness issues across the tour.

Amidst all this, as the older legendary set is reaching its twilight, the youngsters haven't risen dominantly to challenge Djokovic yet. Dominic Thiem and Sascha Zverev are bright faces making their announcements to the tennis world but they have to find their own consistency and develop a lot more to challenge for the throne on a regular basis.

Djokovic looks settled for now and his significant rise has only powered him forward ahead of the others.

Conclusion regarding these factors: If Djokovic continues on this path without external factors affecting his emotions a lot, he can come really close and might as well overtake Federer's Grand Slam tally. Time will tell.