5. Del Potro
I wouldn't be surprised to see a year similar to this year with Djokovic and Murray swapping the #1 and 2 spot a couple of times and it coming right down to the wire to know for sure who will end the year at #1. I think Federer and Nadal could go either way, and a lot will depend on whether or not Nadal comes back at 100%. If he does, anything goes.