We study single-particle inclusive (1PI) distributions in top-quark pair production at hadron colliders, working in the highly boosted regime where the top-quarkpTis much larger than its mass. In particular, we derive a novel factorization formula validin the small-mass and soft limits of the differential partonic cross section. This providesa framework for the simultaneous resummation of soft gluon corrections and small-mass logarithms, and also an efficient means of obtaining higher-order corrections to the differential cross section in this limit. The result involves five distinct one-scale functions, three of which arise through the subfactorization of soft real radiation in the small-mass limit. Welist the NNLO corrections to each of these functions, building on results in the literature by performing a new calculation of a soft function involving four light-like Wilson lines tothis order. We thus obtain a nearly complete description of the small-mass limit of the differential partonic cross section at NNLO near threshold,missing only terms involving closed top-quark loops in the virtual corrections.