Slate's feature on doping in tennis entitled, "Steroids Anyone," doesn't have any bombshells because there has been so little testing. They do cite Jim Courier, who, in 1999 thought EPO was a problem at the top level.
Here's the final paragraph, "Why does tennis remain so lax when it comes to drug testing? Perhaps it's taking a lesson from the other major sporting event taking place this weekend: the Tour de France. Cycling, baseball, and track and field are the sports that have been the most stained by drug cheats. They're also the sports that have the most-rigorous testing programs. Based on recent evidence, it's not realistic to expect top-flight athletes to be clean. It is possible, though, to simply sweep the syringes under the rug—or better yet, not look for them in the first place."