I had a realization recently playing tennis - quit trying to hit like Roger Federer or Rafeal Nadal every time, it's going to wear out your arm.
I had a match a while ago against a slow hitting grinder. He returned nice deep loopy balls back to me all the time, while I try to hit hugh forehands through him with all my force. I won the first set smoothly, but a couple of mishaps on my part and we were 5-5 in the second set. It was then I realized my right arm was worn out. I couldn't go for winners anymore: trying to do so would likely result in an error due to the lack of strength in my arm to control the racque - this is not the first time this has happened either: I have played this same player before and it resulted in a thrilling third set tie breaker with me lossing mostly due to my losing my forehand. I still won this time though by a bit of luck while playing more defensively at the end. At the time I thought: I need much much more endurance.
But now I realized the bottleneck is not the endurance, but rather that I've been hitting too many unorthodoxed - though pretty - shots - trying to mimick Roger Federer or Rafeal Nadal(mostly Roger). A lot of times I would hit the ball on my back foot; many times when I have a slow ball I would wind up my whole body to jump into the ball on the swing(like the pros). These shots, while good looking, and can be effective, can also be extremely demanding on your arm and even the rest of your body.
But more recently I have come to embraced a much more energy efficient, and possibly more textbook style of hitting the forehand. Swing the racque the way you throw a punch in Taekwondo, i.e. start with a wide stance(but usually facing about 90 degrees away from the opponent's baseline rather than being parallel to it), when you hit the ball, lead in with a hard turn of your hips, which is helped with a push from your back leg. This, oh this, is the shot of optimal power generation. I have been using this shot more now in my matchs, and time and time again it has proven worth while. Not only does it take less force to generate more power, I also hit it with more consistency than with the unorthodoxed shots.
How did I discover(rediscover) this shot? We bought some foam training balls on the suggestion from a friend - who is a 4.5 level player. He said he bought the training balls to teach his son, but found it was too early to teach his son and so practiced with the balls himself instead. He found it to be enormously helpful to his game. The foam ball travels much slower than a tennis ball, and so it is very hard to hit a foam ball out of bounce. Thus, when you are hitting with a foam ball, you tend to excert much more energy hitting the ball to make it travel farther. Now, when you have to consistently hit with more power, the natural thing you body and mind tend to do is find the most energy efficient way to do it(cause you get worn out quick otherwise). And that's basically how this shot was rediscovered.
As for Roger and Rafa? They both have a level of endurance that is just incredible. I don't think I would be getting there any time soon.