The knowledge to the increase of contact area prior to gross sliding is a pivotal issue to advance the contact mechanics and nanotribology. This study performed an in-situ observation of lateral junction growth at the nanoscale, for the first time, by nanoindentation equipped in a transmission electron microscopy. The nanoasperity was prepared by focused iron beam of copper coated substrate. From the results of nanoasperity with single crystalline, it was found that the lateral junction growth is presented only when the contact interference exceeded a threshold value, which the full-stick condition was achieved. The observed directions of slips of atoms, slip plane, and movement of the dislocations inside the compressed single crystal copper nanoasperity consisted with the theoretical predictions with applied normal and lateral force. An atomistic simulation with three dimensional flat-on-asperity contact model was performed to provide the experimental results a theoretical interpretation. The experimental results were compared well with simulation results which provided constructive insights into the lateral junction growth phenomena for theoretical prediction. Our experimental and simulation results uncovered the atomic origins of the lateral junction growth at the nanoscale.