[marceltribo]

Depends on your goal.

If there is any need to build up muscles, go with traditional back exercises like Pull-Ups, Dead-Lift and rowing.

If you are looking for sport-specific-exercises, every Core-Exercise is recommandable.
This is where you get to strengten your Core as a whole, which will give you greater stability for your back as well.
Traditional exercises could be a plank, wether straight or for one side. To add a little more dynamic, russian twist is an exercise with great table-tennis-specific.

Most important thing is to make sure your form is good to avoid any injuries and to perform those exercises regularly.