The extended state observer (ESO), as a special case of high-gain observers, can estimate not only the state variables but also unknown disturbances. ESO works well for slowly time-varying or low-frequency disturbances. To extend its performance to high-frequency disturbances, this paper proposes a phase compensator to recover the inherent phase loss in the traditional ESO. Furthermore, the ESO gains are designed as nonlinear functions to reduce the peak phenomenon in linear high-gain observers. Based on the proposed ESO, a frequency-shaped sliding mode control algorithm is designed to further suppress high-frequency disturbances. Simulations are performed on a hard disk drive to demonstrate the effectiveness of the proposed algorithm.