The Hitch said:
Nadal is much much much fitter than all his competitors, and the commentators put this down to him training harder. (now where have i heard this excuse used to defend those who are doping

)
He looked old for an 18 year old when he burst onto the scene.
People have been caught in the lower levels, which suggests people are doping in the higher levels, and if anyone is doping nadal fits the bill.
Drug tests are infrequent and very easy to get around in tennis
nadal turned pro and burst on to the scene at 15 when he beat federer at key biscayne that year--but yeah he looked pretty mature then too (except that in real life, he is really nowhere near as big and beefy as he looks on tv).
as i said in that other thread, players in the top 20 have been busted too, and agassi had rumors swirling around him forever (and since he admitted to being a meth-head, anything is possible).
tennis operates much differently than cycling, and while i would not put it past the ATP to do gentleman's agreements when positives are uncovered from stars' samples (you know, let them know they know and try to put the fear of god into them that if it happens again they will not hesitate to expose them); there is no omerta in tennis among the players. these guys are not friends and do not share an esprit d'corp. if roddick found out that federer was doping when feds beat him in the fifth set last year in the wimbledon final, i have no doubt rodick would beat the crap out of him in the locker room and disparage him in every interview from then on out. however, i don't think th ATP would protect anyone other than say federer, nadal, roddick, or murray--when daveydenko was 4th in the world they had no problem letting the world know he was being investigated for match fixing--which in tennis is a way bigger sin than doping: doping may give a slight edge, endurance-wise and mostly on clay courts, but match fixing goes to the heart of the game.
some of us think that the ATP gave nadal the little drug talk last year and that was why he developed a mysterious knee ailment that ruined the later half of his 2009 and beginning of his 2010. and they have him on short leash this year: he was fined for coaching during wimbeldon and called for time, which almost never happens to the stars.