A novel output-feedback tracking sliding mode control strategy is proposed for a class of uncertain multi-input-multi-output (MIMO) systems with strong nonlinearities and unknown high-frequency gain (HFG) matrix, that is, the control direction is assumed unknown. A switching scheme based on a monitoring function is designed to handle the control direction uncertainty. The proposed method provides global stability properties and exact output tracking. Simulation results about a robotics visual servoing system using a fixed but uncalibrated camera illustrate the robustness and practical viability of the proposed scheme.