I think it's a combination of everything. the most important part is mental IMHO. When I see Djokovic or Fed showing up on the court it's like I see the force, I see so much pure confidence.
When I see these youngsters I'm like 'what the heck'. I joke often about Tomic, Janowitcz but I really wish them well. I just think that somehow they, instead of working hard, like talking too much. they are just so immature. these young guys pretend they are major stars and they are not. I don't see that kind of behaviour from the top 10 guys.
Raonic and Dimitrov seem to really normal regular guys, but they both need to improve so much. I don't really care that much if Djokovic wins or loses IW/Miami but that choking from Dimitrov against him was beyond this world.
same old story, youngsters are not winning anything. Nole won the AO and Dubai, Nadal IW, Murray won Miami ... watch Rafa probably winning everything on clay again. I'm getting bored