1) Get a controller (unless you're very skilled with a keyboard). It makes things much easier. I know most PC controllers are junk, but you don't need a special one for this game. Plus, if you're already a Playstation 2 or XBox owner, there's USB adapters available for very cheap (less than $10 US).
2) Playing offline does not help your online game much. While the AI for Tennis Elbow is quite good, playing against real players is much different.
The biggest difference is serving and returning. Against the computer, you can hit strong, flat serves. But online, unless you're playing in free mode, which most veterans never do, you're unable to hit strong serves.
For returning, the computer even on the hardest levels, their serves are not very accurate. Online, many players are capable of hitting the corners on a consistent basis. They also know how to spin serves effectively, which the computer also rarely does. The only way you'll get a feel for returning is by playing online. Don't give up on it.
3) Stop trying to be Andre Agassi, and taking balls inside the baseline. It doesn't work. All I have to do is hit a loopy, deep topspin shot, and you'll hit that ugly shot that doesn't reach the net.
4) Learn to use all the shots. This is why I think Tennis Elbow is a great game. There's strategy on when to use lobs, drop shots, short slices, deep topspin. If you're constantly hitting power shots, you'll not playing the best you can.
5) "Leelee, how do you have 100% forehand and backhand in fair play mode?"
Just edit your player how you want.

This will probably sound very confusing, but I'll explain the best I can.
In fair mode, you can have an average of 75% in all your stats, with a high of 75% for your serve (which is unnecessary since the highest your serve can be is 50%, I'll get into that later).
The setup I use for fair mode is 100% on my forehand, backhand, form, and service. 63% on forehand volley. 62% on backhand volley. 0% on smash. Now, if you're a net player, you probably shouldn't use those stats, and edit accordingly.
I'm guessing most of you put 100% on all stats, which will drop all of them to 79% or 80% due to the average.
As for the serve, the game separates both your 1st and 2nd service. The most you can put into your serve is 100%, so the best you can get is 50% on each serve.
I hope I help someone.
