you would no doubt agree that Nadal isn't the only tennis player on a heavy duty program. but PEDS do seem to favor his style, or rather generally the spanish style of playing, which is traditionally based on heavy topspin, lots of running, etc (the gravel tradition). The game of guys like Nadal and Ferrer is based almost exclusively on the physical intimidation, outpowering and outrunning of their opponents.
Of course they aren't the first in that tradition (just think of Thomas Muster, Michael Chang, Leyton Hewitt, and some other guys), but Nadal, Ferrer and also Djoker are setting new standards in terms of fitness.
Now, with huge guys like Berdych, Del Potro and Murray running up and down the court like Michael Chang used to do, there can be no doubt that PEDs are a major asset of every top 20 player's game.
I also find Germans Haas and Kohlschreiber somewhat suspect, not just because they're extremely fit, but also because both ignored the olympics in spite of being in topform.