I personally doubt if Djoko is doping. Of course i dont know him or his team personally, so i can't say for sure. But from what i've seen and read around the interweb - he really does train very hard, and makes alot of sacrifices diet wise to stay in top shape. Same goes for Murray.
Not everybody is as blessed as Sampras and Federer with that freakish natural ability - most have to work much harder at it. Tis not to say they are doping or they aren't, i guess time will tell, or it won't