The application of a high electrostatic field at the apex of monocrystalline diamond nanoscale needles induces an energy splitting of the photoluminescence lines of color centers. In particular, the splitting of the zero-phonon line of the neutral nitrogen-vacancy complex (NV) has been studied within a laser-assisted tomographic atom probe equipped with an in situ microphotoluminescence bench. The measured quadratic dependence of the energy splitting on the applied voltage corresponds to the stress generated on the metal-like apex surface by the electrostatic field. Tensile stress up to 7 GPa has thus been measured in the proximity of the needle apex. Furthermore, the stress scales along the needle shank inversely proportionally to its axial cross section. We demonstrate thus a method for contactless piezo-spectroscopy of nanoscale systems by electrostatic field regulation for the study of their mechanical properties. These results also provide an experimental confirmation to the models of dielectrics surface metallization under high electrostatic field.