Hello,
the lower your opponent's difficulty is, the slower he hits the ball.
Thus said, if everything seems too fast, it may mean you need to activate the Game mode on your TV. Otherwise, there's a big input lag, making TE4 (and other action games) pretty hard to play.

And to avoid being aced too often, read the point 3 here :
topic8-6180.php .
Additionally, you change this in your Tennis.ini, under [Game], from :
AiSlowerServer = 0
to :
AiSlowerServer = 20
to make the CPU serve 20% slower. (HowTo =>
topic19-32161.php )