As expected Federer will have one last big run before he retires, its difficult to see him repeat 2012 in 2013 even if he wins few slams here and there, that makes the other 3 fight for the pride again. Rafa for some reason looked knackered to me except for the FO, Djoko looks like a spend force already and Murray is the only one I see as improved his game from 2011.
So the interesting question is, if Murray makes no.1 do you guys think he will hold on to it longer than the other 2 [Nadal/Djoko] did? or will he last the similar amount of time like he lasted in No.2? Slams might be a bit tougher for Murray, but like we saw this year Federer achived no.1 more with success in masters and 500's than too many slam wins, which Murray can certainly do it. Four-Five masters win coupled with 4 semi-finals in slams will give him close to 8k points and wins in 500's and 250's and WTF can help him rack up points close to 10k and if the other 3 battle slams in proportional manner, Murray can easily get to no.1
I think if and when Murray makes the top spot he will cling on to it longer than expected of him, just for the reason he knows how difficult it was to get there and he won't hence goof it up easily. I see Murray handling pressure of expectation better than the other 2 [Rafa/Djoko], I tip Murray to take the no.1 with a Slam title [like Fed did] next year ,USO will be the likely case.