The susceptibility of OCTG-grade alloys to sulfide stress cracking (SSC) in hydrogen sulfide (H2S)-rich sour environments poses a unique challenge for downhole oil and gas exploration, particularly when considering the role of residual stresses. These stresses, inherent in materials from fabrication processes, can strongly influence their cracking resistance even in the absence of external loads. This study numerically examines the effect of residual stresses on the fracture toughness (KISSC) measurements associated with SSC resistance of a high-strength low-alloy carbon steel C110 in H2S-containing aqueous test solution using industry-standard single-edge notched tension (SENT) and double-cantilever beam (DCB) testing methodologies. Residual stresses are measured on pipe samples and are incorporated into the finite element model of the pipe through a thermo-mechanical equivalent loading. The residual stress field is then mapped onto the fracture mechanics test specimens to represent the initial stress distributions. A coupled deformation-diffusion phase-field framework is implemented in COMSOL to simulate crack propagation under the combined influence of residual stress and environmental factors. The results offer insights into SSC mechanisms, demonstrating that elevated residual stress levels in both SENT and DCB tests affect SSC initiation thresholds and arrest, signifying reduced fracture toughness and increased susceptibility to SSC. This study underscores the importance of considering residual stresses in fracture-mechanics-based SSC integrity assessments to improve the reliability of components in sour service applications.