Thanks to Jon and Ouvinen, I tried the technique described in 'Flash in Sports Photography'. I'd like to get your feedback. You can get a better quality of the shot on Photo Sig.

The flash exposure looks good. Now I think you should go for a lot slower shutter speed. Like 1/15s or something. This way the background comes out too sharp. The orange tone of the artificial light in the skate hall is a nice contrast to the daylight colour of the flash.

Yeahhhhhh!

That came out good. The background is darker so your skater is popping out really well. I agree with Ouvinen. Try a slower shutter speed. Motion blur rules!