@ Eddster. I have thought this for a very long time. So have many others but cannot say anything. This is becoming a big problem in my opinion.
I'm glad people are raising this question. When we ask why Andy cannot last as long we may be asking things the wrong way round. How come Novak an can seemingly outlast everyone?
There may be nothing untoward, but some things seem borderline impossible, remembering here the 5 hour win in AO 2012 against Andy, then a 6 hour win against rafa.