How to perform an Ace in Tennis in Grand Theft Auto 5?
Solution 1:
When you serve from the left side of your field to the right side, stand at the most left point of the field, aim to the most right side of the other field and try to get the full power.
It's hard to do an Ace, but it is possible. For me, this way is the easiest way!
Solution 2:
I've seen aces even in my first tennis match, without any strategy what so ever. I wasn't even aware that you can aim you ball.
Just hit the ball at the maximum swing and chances are you'll hit an ace.