El Niño–Southern Oscillation (ENSO) exerts a profound influence on global climate, yet predicting ENSO more than one year in advance remains challenging. Tropical inter-basin interactions have been recognized as an important source of extended ENSO predictability, with the Indian Ocean Dipole (IOD) and the winter Atlantic Niño identified as promising long-lead precursors. Because the IOD also influences the subsequent winter Atlantic Niño, it may affect ENSO through both a direct Pacific pathway and an indirect Atlantic-mediated pathway. Here, we combine observational analyses with targeted climate model experiments to compare the predictive roles of the IOD and the winter Atlantic Niño and to quantify the relative contributions of the two IOD pathways. We find that the winter Atlantic Niño alone exerts only a weak influence on tropical Pacific sea surface temperature anomalies during the following year. By contrast, the IOD exerts a much stronger influence on subsequent ENSO evolution, primarily through its direct impact on the tropical Pacific, whereas the Atlantic-mediated pathway contributes only marginally. These findings identify the IOD as the dominant tropical inter-basin precursor for predicting ENSO beyond one year.