Can i still play tennis with a bruised knee cap?

Probably: If it is truly a bruise and there is no limitation to knee extension then gradual return to activity would be acceptable. I would see your physician to confirm first that the knee cap is not fractured and that your knee exam is normal prior to gradual return to activity.

Patellar contusion: Ok to progress as you gain painless range of motion. Consider naaids, open patellar sleeve or lateral support, laser therapy, purified omega, weight control, good quad & ham flexibility, cross training.