If I have to bet, I would bet 德约科维奇 or 纳达尔 drops faster than 费德勒 because 费德勒 has a very different style than many tennis players; he doesn't rely very much on power and therefore aging would not have much negative effect on him as on those players who relies on power very much. You can see this from the fact that 费德勒 seldom has injuries.