Transient impact-sliding contacts between the cage and the guiding ring under starved lubrication severely threaten the structural reliability of high-speed aero-engine main bearings. Traditional analytical and implicit finite element methods struggle to capture the highly nonlinear competitive failure mechanisms at this dual-body interface due to severe convergence bottlenecks and non-realistic single-body simplifications. In this study, a novel, fully coupled thermo-elasto-plastic phase-field fracture framework is proposed to systematically investigate the asymmetric damage evolution between an M50 steel ring and an AISI 4340 steel cage. The simulation results reveal a profound synergistic coupling effect between the normal impact and tangential sliding velocities. Furthermore, a fundamental disparity in spatial damage morphology is observed: the high-hardness M50 ring exhibits surface-initiated, elastic-dominated incipient micro-damage localization, whereas the tougher 4340 cage accommodates kinetic energy through subsurface-initiated, volumetric, thermo-elasto-plastic flow. Crucially, an asymmetric competitive degradation mechanism governed by an interfacial thermal partition coefficient is uncovered. By purposefully channeling a substantial fraction of the frictional heat into the 4340 cage, the cage undergoes catastrophic thermal softening and massive yielding, autonomously acting as a thermodynamic sacrificial buffer. This mechanism dynamically absorbs the destructive impact energy, physically shielding the core M50 ring from severe fatigue fracture. The extracted damage transition phase diagram provides a robust, physics-based criterion for the advanced surface engineering and thermal management of aerospace bearings.