Hello,
I start with a copy/paste :
The short strike & the short accelerations are meant to do passing shots, which is the 1st way to pass a player at the net ; the normal acceleration is also good when you want to do a passing down the line.
The defensive lob rarely works in-game and in real life. The attacking lob however can be a winner when your opponent is close to the net, so pay attention to his position.
You'll find more details here :
topic15-33305.phptopic15-9763.php
I'll add : your 1st strike is the most important. Your best occasion is when your opponent starts to run from the baseline. If you notice it and send the ball far from him, you'll either do a winner or force him to do a poor shot that will allow you to do a passing winner on the next strike. If your 1st strike isn't good, you can consider you have lost the point and if you end up winning it, that's a good performance on your part.

Globally, to pass the CPU efficiently, you need to be able to aim close enough to the lines consistently and move up/down to get the ball at its highest point.
Also, the short strike is not good when the ball is low and/or hit after the baseline, as it produces a too-slow and high ball.
If you manage to lower your opponent's net success ratio to less than 60%, you'll be already doing well. (when your skills and his skills are roughly equivalent)
I just checked that with the Pro-10 Sampras with Rank #1.
Against what difficulty level are you playing ?