Transition metal chalcogenides with lamellar structure are known for their use in tribological applications although limited to vacuum due to their easy degradation in presence of oxygen and/or moisture. Here we present a tailored WSex coating with low friction (0.07) and low wear rates (3×10-7 mm3 Nm-1) even in ambient air. To understand the low friction behaviour and lower chemical reactivity a tribological study is carried out in a high vacuum tribometer under variable pressures (atmospheric pressure to 10-8 mbar). A detailed investigation of the film nanostructure and composition by advanced transmission electron microscopy techniques with nanoscale resolution determined that the topmost layer is formed by nanocrystals of WSe2 embedded in an amorphous matrix richer in W, a-W(Se). After the friction test, an increased crystalline order and orientation of WSe2 lamellas along the sliding direction have been observed in the interfacial region. Based on high angle annular dark field, scanning transmission electron microscopy and energy dispersive X-ray analysis, the release of W atoms from the interstitial basal planes of the a-W(Se) phase is proposed. These W atoms reaching the surface, play a sacrificial role preventing the lubricant WSe2 phase from oxidation. The increase of the WSe2 crystalline order and the buffer effect of W capturing oxygen atoms would explain the enhanced chemical and tribological response of this designed nanocomposite material.