<p>This study investigates the output tracking control problem with prescribed transient performance for lower-triangular nonlinear systems in the presence of unknown nonlinearities. Unlike existing approaches, the lower-triangular nonlinear systems under consideration exhibit singular input-output links, which are inherently not feedback linearizable. This general characteristic renders conventional techniques, such as integrator backstepping and the method of adding one power integrator, inapplicable. To overcome this challenge, a novel funnel control scheme is proposed, integrating bilateral barrier functions (BBFs) with the definition of a limit. Within this framework, BBFs ensure that the output tracking performance satisfies predefined transient specifications despite unknown nonlinearities, while the definition of a limit effectively handles difficulties arising from singular input-output links. A distinctive feature of the proposed method is its capability to accommodate control coefficients that cross zero during system evolution, a feature not supported by existing techniques. The effectiveness and practical applicability of the proposed method are demonstrated through numerical simulations and real-time experiments on a Franka Emika Panda robotic arm.</p>