The friction force is usually backward, reducing the horizontal component of the ball's forward velocity. (scientificamerican.com)
In that case, the friction force on the ball is in the same direction as the horizontal component of the ball's velocity. (scientificamerican.com)
Therefore, if the topspin is large enough so that 2πRF is greater than Vcm, the horizontal component of the ball's velocity will increase after the bounce. (scientificamerican.com)