[en] This article presents an improved observer-based robust adaptive control strategy for a class of nonlinear systems that have two features: (1) the coupling of unmeasured states and unknown parameters exist at the measured states dynamics and (2) unknown deadzone exists at the system actuation. At first, the bounds of the deadzone parameters are assumed to be known and a suitable control strategy is derived. This strategy involves the derivation of a control action, observer of the unmeasured states, and estimators of the unknown parameters such that global stable system performance is guaranteed. Then, another control strategy is proposed when the bounds of the deadzone parameters are unavailable. The second control algorithm comprises the derivation of suitable control action, observer of the unmeasured states, and estimators of the unknown parameters such that global stable system performance is assured and the anonymity of the bounds of the deadzone parameters is accommodated. Simulations are performed when using both control strategies for a nonlinear system that falls in the category of systems addressed in this article. The system is a single-link mechanical joint that suffers from friction torque, modeled by LuGre friction model, with unknown deadzone exists at its actuation. The simulation results show the high performance of the suggested control approaches.