Drag is the fourth of the major forces of flight. It is a resistance force and slows the forward motion of an object, including airplanes and tennis balls. There are four types of drag: friction drag, form drag, induced drag, and wave drag. The sum of all four make up the total drag force.
Let's look at each type and see whether or not our tennis ball is affected.
For an aircraft, the drag forces oppose thrust. If the thrust force is greater than the drag force, the plane goes forward; but if the drag force exceeds the thrust, the plane will slow down and stop. For our tennis ball, there is no thrust. Yes, the ball had an initial velocity when it was struck by the racquet for .004 seconds, but there's no engine or propulsion system attached to the ball is there? So our tennis ball like a glider (an aircraft without an engine) is going to slow down until it hits the ground or your opponents racket.
The friction drag, sometimes called the skin friction drag, is the force created by surface roughness. To minimize friction drag on an aircraft all the sheets of metal on the wing join smoothly, and even the rivets are rounded over and as flush with the surface as possible. Even dead insects and dirt on a wing cause this type of drag. Swimmers shave their bodies to reduce friction drag. For our tennis ball, clearly the felt covering is very rough and a great source of friction drag. Wind tunnel tests that the ITF (International Tennis Federation) conducted showed that used balls (less fuzz) had less drag than new balls.
The form drag, also called pressure drag , is affected by the shape of the body. A smooth, streamlined shape will generate less form drag than a blunted or flat body. Automobiles are streamlined to increase gas mileage; less drag means less fuel is required to "push" the car forward. You all know the great pains and investment made to streamline race cars. A wing shape is quite streamlined. This is done to reduce the form drag.
Where does the tennis ball fit in? Well, it's blunt, it's a sphere. It's not as good as a wing or an ellipsoid (like a football shape), but its not as bad as a brick. You learned in the wind tunnel section that drag and the drag coefficient Cd are a function the Reynolds number (Re). So let's look at a few Cd's for different shapes. (These are 2D shapes except for the sphere which is 3D.)
Coefficient of Drag (Cd) for Different Shaped Bodies at Re = 4 X 105:
Pigeon: .1
Vulture: .06
Sailplane: .03
Smooth Sphere is at Recrit = Drops From 1 to .3
Smooth Square Cylinder (smooth brick) = 2.1
Induced drag or drag due to lift is a small amount of excess (lift) force generated in the opposite direction of the lift force along a wing. This force slows the forward motion of the airplane and aircraft designers do their best to design wings that lower induced drag. Do we have this effect on a tennis ball? How do you generate lift on a tennis ball? You spin it. So this effect would be present in topspin or backspin but not on a no spin (flat) ball.
The last of the four types of drag, wave drag, generally only occurs when an airplane is flying near (transonic) or faster (supersonic) than the speed of sound (about 760 MPH). Wave drag is caused by the interactions of shock waves over the vehicle and the pressure losses due to the shocks. Since most commercial jets today fly at transonic speeds, wave drag is an important part of the total drag. Okay, Sampras has a fast serve - but its less than 130 MPH. We aren't going to see this effect!
The equation used for drag is:

where Cd is the coefficient for drag, the Greek letter rho (which looks like the script letter "p" is the density of the air, V2 is the velocity squared (velocity multiplied by velocity) and S is the "projected area" (the area that you see when looking at the object upstream). For a sphere, like the tennis ball, S is equal to (pi)r2. Values for Cd are calculated experimentally and generally published in tables. It is fairly typical to look up the Cd value from a table based on either the spin rate or Reynolds number.