<p>This paper focuses on the adaptive prescribed performance tracking control of full-state-constrained high-order nonlinear systems with unknown control directions and unknown time-varying delays. For the unknown system nonlinearities, neither the fuzzy approximation technique nor growth assumption is applied. By introducing nonlinear-transformed functions, combining Nussbaum gain technique and adaptive technique, a novel control algorithm is proposed. In addition, a specific type-B Nussbaum function is added to the controller to avoid the singularity problem caused by derivation. A new exponential lemma based on type-B Nussbaum functions is proposed, which helps us rigorously demonstrate that the closed-loop signals are bounded, full-state constraints are implemented without the feasibility conditions, and the prescribed performance tracking is also realized. The effectiveness of the proposed control scheme is confirmed by the simulation results.</p>