Watch a game from ten years back and compare. It's total madness these days. I'm betting growth hormone.
But if you have watched tennis over that time you can see the gradual progression of speed and technique, and also the height of the players, which is much more than it was.
Regularly an exceptional player comes along that plays at a higher level, and forces other players to move to that level or fall away, and new players that are developing and later come onto the scene are used to playing at that speed and power. Obviously graphite rackets were an enabling technology here.
An example was when Lendl came onto the scene, blasting groundshots all over the place, and then Becker, and then Courier and Sampras, then Federer, then Nadal.
If drugs are being used in tennis they will not be for endurance reasons, they will be to control the state of mind, to try to get the player into 'the zone'. Rudeski always seemed to have a head cold, and say that his coach had given him a concoction of aspirin, etc, but it was probably just to zone him in, caffeine being quite effective at this.
There's a lot of players at similar skill levels and the thing that differentiates them is how they control their head, same goes with golf.
There are loads of highly skilled players that lose to lesser players that have better match heads, otherwise Federer should win all the time, for example.
Where has that Spanish Tennis player gone who used smash everyone?
Suffering from a knee injury, and it was pretty obvious it was going to happen by looking at his goofy style, which relied on his freaky physical attributes. Same goes with Tiger Woods, you could see that he was heading for knee problems from his swing, and he has had to have surgery and change his swing and still hasn't come back properly.
Have a look at photo's of Murray from a couple years back to today.
and then look at how he has gradually changed over time as he has worked in the gym. He has had to bulk up to deal with stronger players, or taller players.