tennis is all about balance.
If you play well, u'd make your opponent play bad, and vice versa.
The main reason there are many difficulty levels, it's coz all people have different levels of reflexes, aiming skill & tactics.
What you probably miss to beat the Master level (which doesn't cheat, except he can aim anywhere all the time) is reflex & aiming skill. Starting to run 0.1s second earlier & aiming a bit closer of the lines can totally transform a match...
The Incredible level has the quickest reflexes possible (around 1/20th of second, sometimes even less), and a 10% relative bonus on all his skills (so 100% for him = 110% for you; 50% for him = 55% for you). But he doesn't have any other bonus, so he gets about as much trouble as u on the rising balls (ie: if he has 90% in consistency & u have 99%, then ur completely equal to strike a rising ball).
It's not forbidden to use the MtRun cheat code, but personally I prefer to compete with the Pro level, which is well balanced for my own skills...
But some people are a lot more skilled than me, and they need greater challenges, that's why Master & Incredible are there..!
In TE2006, some people complained the Incredible level was too easy, so I greatly raised him in TE2009.
Also, I understand u can have fun to try to match the Incredible level using a little speed help...