User: What is a top spin serve in tennis?
Assistant: In tennis, a topspin serve is a type of serve that involves brushing the tennis racket upwards against the ball such that the top of the ball spins forward. This causes the ball to dive down faster than a flat serve. It also causes the ball to bounce higher than a flat serve. Because the server isn't hitting the ball flat and some of their energy is spent adding spin, it is often slower than a hard flat serve.