The Tonga-Kermadec subduction zone exhibits the fastest observed trench retreat (up to 16 cm/yr) and convergence rate (up to 23 cm/yr) near its northern end. However, it exhibits a paradox: despite this rapid trench retreat, the Tonga slab maintains a relatively steep dip angle (53°) above 400 km. The slab turns flat around 400 km, then steepening again until encountering a stagnant segment near the 670 km discontinuity. Despite its significance for understanding slab dynamics, no existing numerical model has successfully demonstrated how such a distinct slab morphology can be generated under the fast convergence. Our mantle convection models successfully reproduced the observed slab geometries while incorporating the observed subduction rate. A key element of achieving a qualitative match lies in the implementation of a hybrid velocity boundary condition, which proves crucial for handling the fast trench retreat. Our investigation explains how the detailed slab structure is highly sensitive to physical parameters including the seafloor age and the mantle viscosity. Notably, a nonlinear rheology, where dislocation creep reduces upper mantle viscosity under strong mantle flow, is essential. The weakened upper mantle allows for a faster slab sinking rate, which explains the large dip angle. Our findings highlight the utilizing rheological parameters that lead to extreme viscosity variations within numerical models to achieve an accurate representation of complex subduction systems like the Tonga-Kermadec zone. Our study opens new avenues for further study of ocean-ocean subduction systems, advancing our understanding of their role in shaping regional and global tectonics.