A roller shade mounting system provides a tube-in-tube arrangement for reducing sagging of a roller tube windingly supporting a shade fabric. The mounting system includes a mounting tube received within a roller tube, and first and second annular drive rings located on an outer surface of the mounting tube. The drive rings are secured to the mounting tube and the roller tube secured to the drive rings such that rotation of the mounting tube results in rotation of the roller tube. Each drive ring is located at a distance from an end of the roller tube for limiting sagging deflection of the roller tube. The tube-in-tube arrangement provided by the mounting system provides controlled deflection for the roller tube without necessitating undesirably large increase in the diameter of the roller tube, which could create an undesirable appearance in many roller shade installations.