Sensitivity has different factors all of which I am not even aware of. I have listed some below. In circus, you may have seen glass ball performing such as here which definitely requires good focus and hand manouvering, perhaps useful for tennis player.
Warming up also the small muscles handling the small weights
Gym training with small weights such as 1kg and 2kg.
Better relaxation such as cold paths.
Playing with smaller balls and balls of different weights
Better hand-eye-coordination