We consider the problem of designing a tracking system for nonlinear affine SISO plants under unmatched exogenous and parametric disturbances and incomplete measurements. In the framework of the block approach, we develop a discontinuous dynamic feedback control law ensuring a tracking with a given accuracy for the output variable of the command signal under the assumption of exogenous actions being smooth. The main result of the paper is a single-parametric procedure for designing a reduced-order observer with high-gain feedbacks for measurement-based estimation of the error of tracking the mixed variables used to form the feedback.