This paper presents a simple generalization of the single-mode Giesekus constitutive model with retardation time. It is based on the utilization of the Gordon-Schowalter derivatives for the stress and rate-of-deformation tensors, which account for the non-affine motion of polymer strands, and make the model more versatile in terms of its ability to predict the complex flow behaviour. The model is consistent with molecular kinetic theory, and has been tested in several well-characterized shear flows, whereas its ability to describe shear-free flows has been proved as well. Finally, a comparison against steady shear data of nearly monodisperse polybutadienes demonstrates its superiority with respect to other continuum or molecular models exhibiting non-monotonic constitutive behaviour.