Nonlinear viscoelastic creep was studied on polypropylene/fumed silica nanocomposites. The free-volume theory of nonlinear viscoelastic creep was successfully applied to obtain generalized creep master curves using a tensile compliance vs. internal time superposition in the region of nonlinear viscoelasticity. Concurrently, a time-temperature superposition approach was also adopted for the construction of creep master curves. A good agreement between the time-strain and the time-temperature superposition approaches was assessed by comparing the master curves obtained from the two data reduction methods. Both approaches evidenced a remarkable stabilizing effect induced by the nanoparticles that was observed especially for higher creep stresses and at increased temperatures and, considering the correspondent superposition principle, at long loading times. At the same time, both storage and loss moduli measured through dynamic mechanical analyses, were enhanced in all nanocomposites. Activation energy values obtained from the analysis of dynamic multi-frequency tests were in good accordance with those referred to creep tests.