We report results from in-situ measurements of lattice expansion during flash sintering of 3 mol% yttria stabilized tetragonal zirconia taken at the Advanced Photon Source, Argonne National Laboratory. The expansion is anisotropic, with the relative expansion of the a-lattice constant exceeding that of the c-lattice constant. The anisotropic expansion cannot be explained by thermal expansion and is consistent with predictions from ab-initio calculations based upon the generation of vacancy-interstitial pairs of zirconium and oxygen.