I've found this video a while back and forgot to upload this. If you remember the highlight from months ago of me finally getting 999 rallies, this is the video that shows it from beginning to end. This is one of the most difficult challenges I've done in a long time.
I've said before there is some sort of strategy to this, but it all mostly comes down to predicting and reacting where the AI will hit the ball back. I've been paying attention to the AI, especially at rallies 280 to 299 where he does the same thing over every 100 times. That is pretty much the only reason why I made it this far.
The best strategy I can say other than react quickly is to always stay in the center after hitting the ball so you have a bit more time to react to where the ball is going to go.