Checked those walkthroughs Lafazar posted. It just said that you should find the correct path to get across but thought that most people will likely understand that.

So for those with no patience for that room I'm offering the paths you should walk to get across to get the tennis racket and back.

To get across press wait until a platform first appears in front of you and then press: Up, Right, Up, Left, Up, Up, Right, Right, Up, Up, Up, Left, Left, Down, Up, Up, Left, Up, Right and Up.

To get back again wait until a platform appears in front of you again and then press: Down, Right, Down, Down, Left, Left, Down, Left, Down, Down, Down, Left, Left, Left, Down, Down, Left and Down.

Pretty sure that's correct, also pretty sure there's other paths and maybe even shorter ones.