Originally Posted by BauerAlmeida
Sounds reasonable. Although I think Fed will end up 4 or 5 this year given he is going to play less tournaments and Nadal will be a bit rusty at the beginning of the season.
The problem is that unless their decline is absolutely massive or they have an extremely long injury spell, only a Del Potro with no health issues for an entire year can threaten their top 4 position in any way - there are more players capable of beating them of course, but not of being consistent enough to overtake them. And we know Del Potro staying healthy isn't exactly a very likely scenario, so Federer and Nadal look likely to keep their ranking even with less tournaments in their schedule and rust.
With that said, Nadal will be in danger if he can't be in form for the clay season, and Federer can hit a significant decline at any time given his age, but they're still safe bets.