Maybe that only matters for playing tennis. These look almost like the design of any other airless tire, only instead of a rubber tire with holes in it they are

Maybe that only matters for playing tennis. These look almost like the design of any other airless tire, only instead of a rubber tire with holes in it they are holding together the tennis balls to form this sort of shape. I think it only matters that they are able to hold their shape up, not that they remain springy or anything.