ManuTOO, I think that this is may be a bug. I can confirm that the option to make the CPU player stronger does not appear, having beaten the CPU 6-2, 6-1, and also 6-0. Attached is the 6-0 match (one great match, by the way
What I notice at the end of this match is that the CPU player changes her tactic; she hits a drop shot and starts coming to the net. This happens often when I am winning, and I love it. It is like this is a tennis match with a real player.