WS2/a-C nanocomposite coatings were deposited by magnetron co-sputtering using WS2 and graphite targets. The microstructure and triboperformance of the coatings were scrutinized via microscopy (AFM, SEM, FIB, HRTEM), spectroscopy (XRD, XPS) and tribometry. Atomic WS2 platelets are randomly embedded in an amorphous carbon matrix of the as-deposited nanocomposite coating. HRTEM observations of tribofilm/transfer layer reveal that the sliding contact immediately reorients WS2 platelets parallel to the sliding interface and thereby leads to self-adaptive “frictionless” response. The coefficient of friction falls to 0.02 in dry air and reaches 0.10 in humid air, and is reversible as testing atmosphere cycles between dry air and humid air.