I doubt fed or djokovic are doing something illegal. However those players whose hair is thinning and falling off are suspicious imo. Some players that this sudden hair loss /thinning is happening to: Nadal, Murray, Gasquet.
All 3 of then are getting male pattern baldness (top of the head), and some really weird looking thin hair. Pay attention to that when cameras are zooming in. It can be a sign of doping.. they are not 45.
I do not see anything on the "outside" for Fed or Djokovic that would suggest steroids.. one can even argue that they are on the skinny side, with no abnormally developed muscles - look at Berdych's and Murray's legs and Nadal's "guns"?
Being an "elegant" mover (very relaxed), allows Federer to be what he is, and nothing tells me that he has some extra strength.
For Novak, it is his overall low body weight (he doesn't have to carry 200 pounds as Murray) that allows him to move fast and stay fresh after 5 hours, and flexibility (doping will not help with being flexible) that allows him to be so successful. There is no doubt that Djokovic is also very fit, but it is not in a way that looks suspicious to me. When he rips his shirt off, he looks like a very fit scrawny guy. What I mean is he doesn't have broad shoulders like Tsonga, nor does he have legs like Berdych, nor arms like Serena / Stosur.
His ability to play 5 hours matches is remarkable, but it is something he sucked at in early days of his career and is most likely what he works most on currently. Coupled with what I mentioned above, doesn't look strange to me at all.
Just my 2 billion cents.