Computational modeling of faulting processes is an essential tool for understanding earthquake mechanics but remains challenging due to the structural and material complexities of fault zones. The phase-field method has recently enabled unified modeling of fault propagation and off-fault damage; however, its capability has been restricted to simplified anti-plane settings. In this study, we extend the phase-field method to in-plane faulting by introducing two key advancements: (i) the incorporation of enhanced fault kinematics and pressure-dependent shear strength for a more accurate representation of fault behavior, and (ii) a revised fault propagation criterion that explicitly accounts for the coupling between shear strength and normal stress. The proposed formulation is verified against standard discontinuous approaches to quasi-dynamic fault rupture under in-plane conditions and validated using experimental observations and numerical data on fault nucleation and propagation. Simulations incorporating structural complexities and material heterogeneities demonstrate the robustness and versatility of the phase-field model, establishing it as a powerful tool for investigating the interactions between fault zone properties and earthquake processes.