Water structure has an essential role in biological assembly. Hydrophobic dewetting has been documented as a general mechanism for the assembly of hydrophobic surfaces; however, the association mechanism of hydrophilic interfaces remains mysterious and cannot be explained by simple continuum water models that ignore the solvent structure. Here we study the association of two hydrophilic proteins using unbiased extensive molecular dynamics simulations that reproducibly recovered the native bound complex. The water in the interfacial gap forms an adhesive hydrogen-bond network between the interfaces stabilizing early intermediates before native contacts are formed. Furthermore, the interfacial gap solvent showed a reduced dielectric shielding up to distances of few nanometres during the diffusive phase. The interfacial gap solvent generates an anisotropic dielectric shielding with a strongly preferred directionality for the electrostatic interactions along the association direction.