A theoretical model has been proposed to assess quantitative residual stress from a stress-induced shift in an indentation curve, but the assumption in this model of equibiaxial surface stress has obstructed its application to real structures in complex stress states. Thus we investigated the influence of non-equibiaxial surface stress on contact deformation through instrumented indentations of a biaxially strained steel plate in order to extend the model to a general surface stress by considering a ratio of two principal stresses.