Personally, I think doping, steroids, etc. must be rampant at the top levels of the tours, especially in the ATP. I know those bill goats are in phenomenal shape, but to win a Grand Slam, especially the French, requires endurance the likes of which is rarely seen in sports. Off the top of my head I think only the Tour de France is more strenuous. Soccer and NBA basketball might be up there, too, but only biking seems tougher. And I'm sure doping in biking is more common than we're led to believe.
In general, I think drugs and doping are used a whole lot more in professional sports than they let on, and in some sports it might be common practice. Those athletes have every reason to use them. Who knows how long you're going to last in pro football or baseball, so why not use some drugs like everybody else is doing, get those numbers up, and make as much money as you can? Same for tennis. Same for the Olympics. I'm sure "everybody" is doing it. There's just too much money at stake, and in some cases history as well.
I think the women do steroids for a strength advantage, whereas the men do it to speed up recuperation. It may just be common knowledge in the locker room that everybody partakes in a bit of illegal substance usage. I wish there was a website where you could get more of the inside dirt. I remember that French player who made waves with her tell all book insinuating that Capriati took steroids in order to fuel her super comeback. Her arms were looking awfully big. Hmmmm