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This paper considers the problem of robust output tracking control for multi-input multi-output (MIMO) nonlinear systems in the presence of mismatched time-varying uncertainties. Using singular perturbation method and variable structure control technique, we derive a new hybrid controller for MIMO systems with unknown time-varying uncertainties and disturbances to track a desired trajectory. It is shown that the hybrid controller not only stabilizes the closed-loop systems, but also guarantees the tracking errors remain in an O(ε) neighbourhood of the origin where ε is a small design parameter of the controller. Moreover, by selecting the parameter ε properly, the tracking errors can be made arbitrarily small.