This paper presents a solution to the problem of trajectory tracking with external disturbance attenuation in robotics systems via a reduced-order output feedback controller, without velocity measurement. The proposed control law ensures both semi-global asymptotic stability of the closed-loop system and external disturbance attenuation. The approach is based on the notion of the L-2-gain and requires to solve two algebraic Riccati inequalities. The proposed controller design is illustrated by a simulation example.