The problem is you must apply tension to the strings, before the ball hits the racket.

Have you looked at a tennis racket frame and the strings?  There are holes in the frame and the strings go though the holes. Create holes in the frame. Investigate using a Translational joint between the vertex on the end of the string that ends at the center of the hole and the sides of the hole. I'm not sure about Explicit Dynamics, but if you use Transient Structural, you could have Step 1 use a Joint Load to apply tension to the string, then bounce the ball off the string in step 2.