High-precision measurements of joint displacements and elastic forces are available on a flexible joints manipulator. In contrast, velocity measurements and elastic forces derivative are, in many cases, contaminated by noise or unavailable (technical or economical constraint). It is, therefore, interesting to investigate the possibility of controlling robot dynamics by only using position and elastic force measurements. This paper presents a singular perturbation approach for trajectory tracking control combined with nonlinear sliding state observer. The stability of the closed-loop system is investigated.