To jump in the doping fray: the only way to catch steroid or hgh use is through extensive out of competition testing. Athletes take these drugs in training to build muscle, not during competition.
The ATP only tests players when the leave a tournament (i.e. lose or win the whole thing). Many drugs such as low doses of EPO are only detectible for 24-48 hours. This means that any of the top competitors can dope without fear in the early rounds of the Majors to be fresher later (unless you lose to a low ranked player in the second round, hence the rumors that Nadal got caught doping and his knee injury was to cover the secret suspension—since adverse doping findings are kept secret until a player is determined to have doped).
If the ATP was serious about combating doping they would institute a biological passport and continually monitor the top 150 year round. Not that the biological passport system is perfect, but it vastly limits the amount of doping.
Additionally, what is with all these asthmatic super athletes like Serena Williams that must use an inhaler every time they change sides?