Y’all are ignoring any curvature in the flightpath from the spin of the ball and the effect of the difference in air speed on opposite sides of the ball. As has been noted, acceleration is a change in speed or direction.
So to summarize for the OP who is now probably totally confused: the tennis ball will begin to slow down as soon as it loses contact with the racket, but there will be some second order effects due to air resistance and gravity accelerating the ball downward.
True, but it does cause acceleration after the ball has left the racquet, and that in turn increases or decreases the distance the ball travels. In keeping with the spirit of the OP, if not the letter
Being smashed by the racquet and then rebounding to a more-or-less spherical shape would cause the tennis ball to ‘ring’ after it leaves the racquet surface. This ‘ringing’ would not affect the velocity of the center of mass of the ball, but the velocity of a point on the leading edge of the ball would oscillate around that mean value. So, if you were just measuring the velocity of the leading edge of the ball, it would probably continue to accelerate for a fraction of a second after leaving the racquet surface - then it would appear to slow down, then speed up again, etc. until hysteresis damps out the ball’s oscillation.
