The velocity is increasing in the downwards direction and the acceleration is increasing in the upwards direction.
There are 2 forces acting on the tennis ball, gravity and air resistance.
Gravity is treated as a constant force in the downwards direction.
Air resistance is a force that acts in the opposite direction of the ball's movement relative to the air.
The air resistance cannot be greater than the gravitational force if there is no wind.
The sum of these forces is directly proportional to the acceleration of the ball as given by Newton's second law
Since gravity is larger in magnitude than the air resistance, the ball is always accelerating downwards, and hence its velocity is increasing in the downwards direction.
To determine its change in acceleration, we have to examine how the forces acting on the ball varies with time. Gravity remains constant while the air resistance increases as the ball speeds up. Thus, the total force acting on the ball decreases with time and consequently the ball's acceleration decreases as well.