Nanoscale metallic crystals have been shown to follow a 'smaller is stronger' trend. However, they usually suffer from low ductility due to premature plastic instability by source-limited crystal slip. Here, by performing in situ atomic-scale transmission electron microscopy, we report unusual room-temperature super-elongation without softening in face-centred-cubic silver nanocrystals, where crystal slip serves as a stimulus to surface diffusional creep. This interplay mechanism is shown experimentally and theoretically to govern the plastic deformation of nanocrystals over a material-dependent sample diameter range between the lower and upper limits for nanocrystal stability by surface diffusional creep and dislocation plasticity, respectively, which extends far beyond the maximum size for pure diffusion-mediated deformation (for example, Coble-type creep). This work provides insight into the atomic-scale coupled diffusive-displacive deformation mechanisms, maximizing ductility and strength simultaneously in nanoscale materials.