You are right. In an optimal system, there isn't any friction. But in real life there istis, cause abecause there aren't any balls or groundssurfaces that are so perfect, that they touch just in one point.
Also the surface of the ball is never perfectperfectly flat, so there will always be friction between the surface of the ball and the surrounding air.
So for calculating the friction, you need to know the exact area in which the ball and the ground touches. And you have to include the friction between the moving surface of the ball and the air.
Sorry for my bad english, hope you understand it nevertheless