I agree that Ivan Lendl is a prime example of taking conditioning to another level to gain advantage. I wasn't around when Borg was playing but by all accounts he was the pioneer of bringing physicality and fitness to the game. He outlasted everyone and is quoted as saying he was never tired on a tennis court. The next player after Lendl that comes to mind who relied on fitness as his weapon was Jim Courier. At his peak he out muscled his opponents. Agassi stated he sought out trainers after loosing to courier to become physically stronger so he could make up ground to compete. Courier was a great player, but but no means was he a technically graceful player. The player of this generation who raised the stakes is Rafa. He is a fantastic talent but his strength and endurance go along way to helping him beat players that are superior in terms of technique. You can also see the top 10 players physical conditioning is superior to the remainder of the top 100. People draw parallels to boxing when talking about tennis. This parallel has never been truer when you look at the state of the modern game. It is very physical. Not only do you have to be great technically, you have to be an elite athlete to be among the top.