The adsorption and desorption of water molecules, which affect the physical and chemical properties of the sliding interface, are critical for the friction behaviors of two solid contacts in atmosphere environment. The amount of water adsorbed on the open surface is a function of gas pressure according to an adsorption equation. However, for a confined sliding interface, the variation of surface fraction covered by gas molecules with water vapor pressure and its induced effects on friction have not been figured out. In this work, the macroscopic friction of diamond-like carbon (DLC) films in a water vapor atmosphere is quantified on the basis of microscopic adsorption and removal of water molecules. The studies correlate the fraction of water molecules adsorbed on the interface of self-mated DLC films with water vapor pressure to illustrate the direct relationship between friction coefficient and gas pressure by first-principles calculations and model fitting. The calculated results revealed that chemisorption and physisorption of water molecules occur on the surfaces of hydrogen-free DLC films (ta-C) and hydrogenated DLC films (HCF). Then, the relation between friction and gas pressure was built by employing a fractional coverage model based on the linear adsorption equation and gas removal. The obtained model agrees well with the typical experimental results about the steady-state friction coefficient of both highly hydrogenated DLC film (HCF) and tetrahedral amorphous carbon (ta-C) film during sliding at various water vapor pressures. In addition, it gave the curves of fractional surface coverage as a function of water vapor pressure. These results show that the frictional coefficient of DLC films could be predicted on the basis of fractional surface coverage as well as the intrinsic characters on surface chemistry. We suggest that the model may be thus extended to understand and predict the friction of DLC films under a specific gas pressure at a low load and speed.