The continuous-time, high-cycle fatigue model of Ottosen et al. (2008) is modified by introducing a quadratic, instead of linear, endurance surface. This quadratic endurance surface induces an elliptical Haigh diagram, consistent with experimental fatigue data for ductile materials. With this modification we fit the model to uniaxial fatigue data, and predict the fatigue behavior for nonproportional, biaxial stress for \mboxAA7075-T6 in fully reversed shear with a superimposed, static tension/compression. However, the fatigue life is sometimes overestimated for a combination out-of-phase shear and tensile stress fluctuations.