Structural and electronic modification of titanium oxide (TiO2) nanomaterials induced by the co-introduction of fully disordered glass phase and oxygen vacancies can lead to remarkable advances in the electrode performance in emerging energy storage systems. We report on the effective co-creation of fully amorphous nanofibers (NFs) composed of black TiO2-x and conductive carbons throughout the NF structure, and evaluate the materials as potential anodes in sodium-ion batteries. The black TiO2-x nanofiber is successfully fabricated by electrospinning a precursor solution followed by a two-step sequential thermal treatment in an air and reducing atmosphere. The NF electrode could deliver approximately two-fold higher 2nd discharge capacity and an excellent kinetic performance even under high rates compared to that delivered by anatase-structured white TiO2 NFs used as reference, because of (i) an inherent free volume in the glass phase corresponding to the enlarged Na+ sites, (ii) increased electrical conductivity (low bandgap) resulting from the presence of Ti3+, (iii) introduction of conductive carbon agents around the TiO2-x domain, and (iv) one-dimensional NF feature allowing numerous Na+ reaction sites at the electrochemical interface. We also elucidate the morphological and structural changes in the nanofibers after discharge and charge by ex-situ characterizations.