fed is the least likely to dope/take any drugs.
nadal is suspicious, murray and joker i think is clean as well like fed.
unlike bike racing and baseball, tennis is not an endurance sport or one that requires hitting the ball really hard, so i dont think dope is that effective (very min knowledge about dope, so maybe i'm wrong)
one thing that lance did was to re-inject his fresh blood to keep him fresh, this is more possible scenario in tennis, however considering the amount of tournaments they played and in different countries i dont see this happening (storing and moving blood package undetected year after year? yeah i dont think so) and even if it happen it'll be on occassion where the person is really tired, wont be on every match, maybe before a GS final or something.