In this paper, a novel design of an Extremum-Seeking Controller (ESC) based on Sliding Mode Control (SMC) for a class of Single-Input-Single-Output (SISO) uncertain or unknown nonlinear systems is proposed. The corrective term of the proposed observer is used to estimate the uncertain system. Global exact tracking using only output-feedback is proven with a tanh periodic switching function. The tanh periodic switching function is shown to provide a smaller vicinity of the desired extremum point compared to the traditional sgn periodic switching function. Numerical simulation results are presented to demonstrate the advantages and effectiveness of the proposed control strategy.