Achieving atomic-scale planarization requires the simultaneous control of interfacial chemical activity and shear-induced material removal, a coupling often beyond the reach of conventional surface finishing approaches. Here, we proposes an external-field-enhanced tribochemical strategy based on cerium oxide nanoparticles subjected to electric and magnetic stimuli. This method enables sub-nanometre surface roughness (Ra = 0.92 nm) through distinct activation pathways. Electric fields enhance electrophoretic dispersion and modestly increase Ce 3+ content (from 27 % to 33 %), while magnetic fields induce Ce 4f orbital spin polarization, significantly promoting oxygen vacancy formation and elevating Ce 3+ levels to 42 %. These field-driven mechanisms modulate redox kinetics, charge redistribution, and interfacial adhesion energy, as supported by tribological experiments and spin-resolved density functional theory. Compared to untreated frictional conditions (Ra = 148.7 nm), magnetic excitation yields a 35-fold enhancement in surface uniformity. This work establishes a tunable, friction-activated interface engineering approach for high-precision applications in optics, microelectronics, and next-generation material systems.