The momentum-space configuration of the participant nucleons in the early stage of heavy-ion collisions can be described by a colliding nuclear matter configuration, i.e. by two Lorentz elongated Fermi ellipsoids. We introduce this configuration in the framework of the Hartree approximation of the σω-model. The configuration and the mean fields are constructed self-consistently, respecting the Pauli principle for interpenetrating ellipsoids at low relative velocity in a covariant and density conserving manner. In a second step we approximately construct mean fields for these configurations in the context of Brueckner theory. To do this we parametrize the real part of the self-energy of relativistic Brueckner calculations for equilibrated nuclear matter (one-Fermi sphere) in a Hartree scheme to obtain momentum- and density-dependent coupling parameters. With these we calculate the scalar and vector self-energy of a nucleon in colliding nuclear matter consistently with the configuration. The mean self-energy components show strong correlation and exchange effects. The results can be used to improve relativistic transport calculations for heavy-ion collisions.