My thoughts about the 3D animation:
First of all, I want to say I think you have done a great job so far. It looks mostly great. But, there are certain minor things that I believe make the animation look somewhat unnatural.
I think the players should have a lot better "anticipation" of the ball...like it should be longer first of all. Now the ball is almost at them when they take the rackets back...also the players run at full speed to the ball and then suddenly freeze and then hit...they should slow down their movement a bit more gradually, and take the racket back early. In real life this action is usually graceful looking, in this animation it looks a bit hasty and the timing looks unnatural to me. I think clearer animation for the position taking would make a big difference.
What makes it look unnatural is that the ball leaves the racket like a bullet, even though it shouldn't. I think if a player moved and took position like in this animation in real life it would result in rather poor shots.
It's really about the "potential energy". It seems as if the players aren't generating much of it to release to the ball for it to be such a bullet, it seems they get the strength from some miraculous source, not from their body. lol
Now the players look somewhat lazy, and they just run to the ball without slowing down to take proper position, it's almost like they don't take position at all, they don't bend their knees enough, they take the racket back way too late, and the racket swing and shoulder turn are a tad bit slow, in real life they are very fast, you can't even see the racket during the swing, nor you can see the shoulders turn.
I do think you achieved all of this pretty well in TE 2006 animation, so if you could do the same or even better with 3D that would be so awesome. lol