Sorry. It is a mistake.
The bottom of the ball tends to moving [b]backward[/b] relative to table when the ping-pong ball touch the table. So the friction force is in the [b]same[/b] direction relative to the horizontal velocity.
Can you see the simulation? Formulas are displayed as java applet. You need to install java run time to view the simulation and those formulas.
The simulation already include the gravity. However, I assume the velocity in the vertical direction only change direction when it bounced on the table (keep the same magnitude to make the calculation simpler!)
Kinetic energy will be transformed into thermal energy if the sliding effect is introduced.