Djokovic has already passed Federer on both year end at #1 (6 v 5) and number of weeks at #1 (323 v 310).
Federer is also losing head to head against each of Nadal and Djokovic.
In the case of Nadal, Federer has basically trailed for his whole career and the score is currently 24-16 in Nadal's favour.
In the case of Djokovic, Federer had the lead early on 4-0 and later 12-6. Djokovic caught up 20-20 in 2015, and it now stands at 27-23. In recent years Djokovic has won 10-3.