This prediction is premature. He is 0-4 against Federer (3 hard, 1 clay), and
1-3 vs Nadal (2 clay, 2 hard). In fact , fed gave him a severe beatdown in AO, and Rafa just put him to pasture in rome. So if Fed stands in his way in hard, and nadal stops him on clay, and he isnt a good grasscourt player, he may have to wait a long time.
and by the way, he and Nadal have one year age difference, Nadal at the same age Djokovic is now had triple the singles titles that Djokovic has now, if we compare them today the number is quadrupled.
My prediction is Fed will come back to form in the grass and hardcourt seasons, and Nadal will continue to dominate clay. He may have to settle for #3