A micro- and nano-structure at the contact interface of triboelectric nanogenerator (TENG) is an essential element enabling a remarkable performance improvement. Despite the various innovative inventions of interfacial structures, there has been little understanding about the analytical interpretation of the contact interface. Herein, the deformation behavior of interfacial micro- and nano-structures during the contact process is systematically analyzed to clarify the origin of the contact pressure to output voltage relationship of TENG. The visualization experiment, simulation, theoretical modeling, and electrical measurements are conducted for the comprehensive analysis. The results confirm that the deformation of interfacial structures directly determines the pressure-voltage relationship of TENG. The surface enlargement effect by interfacial nanostructure formation becomes completely valid only when the provided contact pressure is stronger than the certain threshold pressure to make the full-contact condition. Due to the correlation between contact pressure and output voltage, interfacial structure of the TENG should be carefully customized depending on the application environment. Based on the analyzed pressure-voltage relationship of the TENG, a hybrid circuit for energy scavenging and digital pressure sensing is designed and experimentally demonstrated.