I believe he has a friend in New Jersey who has a 'Pod' which you sit in to help you recover far more easily than ice baths etc. and it also reoxygenates the blood. Seemingly, it is very new and there are very few in the world at the moment.
Also the ITF and ATP haven't yet made up their minds as to whether this is "performance enhancing" and therefore unacceptable to them.
it seems there are some people around tennis who see his run as somewhat suspicious
That was always going to be inevitable. I've heard dark talk about new, highly sophisticated performance enhancing drugs which cannot be detected by the usual drug-testing methods - and I bet he's probably the most tested player on the planet just now, just as Nadal was when there were suspicions about his well-developed muscles.
The much more likely truth is that Djokovic (a) changed his diet after being found to have a sensitivity to gluten, something which can cause a lot of problems, including constant feelings of fatigue due to the fact that this condition stops the nourishment from food from entering the blood-stream; (b) consulted a sports psychologist; (c) worked his butt off; and (d) got a huge boost from Serbia's first win of the Davis Cup, which tie took place in December after