In 2007 after one of the longest matches at the USO, they called Nole “marathon man.” Indeed, he played several long matches at slams that year, especially Wimbledon. 2009 Madrid SF. Djokovic went toe-to-toe with Nadal physically. In 2009, after suffering breathing issues during heat etc, Nole hired Thomas Musters old trainer to see if an improved fitness could help, and then late in 2010 he changed his diet. It’s been a process, it didn't happen overnight. As physical as it is, these guys, in their physical peaks, are more than capable.
But of course some people need to imply doping accusations to diminish his achievements, like this would make their idol look better or something. Making unfounded doping accusations should be a reason for a ban.
And that was all before he found about gluten intolerance. He has been performing at top level with this debilitating ailment for years and then eliminated it with a diet change. It's like he has been playing tennis with shackles on his ankles all that time
and then had them removed all of a sudden. I'm more surprised by the fact that he is still showing signs of tiredness from time to time.