This paper presents a computational study of the role played by surface diffusion on first-stage sintering of powders that densify by grain-boundary diffusion. Coupled grain-boundary and surface diffusion is considered as the mechanism for matter redistribution. By using several novel approaches of presentation of the numerical data, it is shown that the assumption of fast surface diffusion is invalid for typical sintering conditions and materials in first stage of sintering. The study reveals a simple explanation for the role played by surface diffusion in matter redistribution of combined grain-boundary and surface diffusionsurface diffusion changes direction from moving atoms away from a contact neck to depositing atoms onto it as the rate of surface diffusion increases. The reverse surface diffusion blunts the neck and retards densification. It is shown that this mechanism is significant not only for free sintering but also for pressure assisted sintering. It is further confirmed that the widely observed beneficial effect of spark plasma sintering on densification can be, at least partially, attributed to its fast heating rate, which quickly passes through sintering at low temperatures where surface diffusion dominates.