Grain coarsening inhibition in hard metals is regarded as controlled by formation of interface complexions. To date, however, direct experimental insights into the presence and evolution of interface complexions during sintering of hard metals have been lacking. We here present in-situ small-angle neutron scattering (SANS) experiments up to 1500 degrees C and provide first-hand evidence on the thickness and volume fraction evolution of (V,W) Cx interface complexions in V-doped hard metals at various sintering temperatures. The experimental data is complemented by simulations using a thermodynamic-based model to understand the mechanisms behind the nanostructure evolution. We show that there indeed exist (V,W)Cx interface complexions at liquid-phase sintering temperatures; and their thickness and volume fraction are strongly related to the presence of bulk (V,W)Cx precipitation, the V activity in the Co-rich binder phase, and the temperature. The thermodynamics-based model, including the geometry of the investigated material system, reveals that the formation of (V,W)Cx bulk precipitates is energetically favorable over the thickening of complexions in the stability range of bulk precipitation. This, explains the reduction in complexion volume fraction and thickness with increasing temperature up to the dissolution of bulk precipitates. Upon dissolution of bulk precipitates, enhanced interfacial layer formation occurs through the formation of new layers of lower thickness, leading to better coverage of WC grains. The provided understanding of the nanostructure evolution during sintering is expected to foster the further development of representative modelling tools.