(100)-epitaxial SrTiO3 thin films having biaxial compressive strains up to -1.60% were grown on lattice mismatched substrates. Two phase transitions induced by the coupled instabilities (antiferrodistortive and ferroelectric) in SrTiO3 were revealed in a common set of samples investigated at temperatures from 8 to 600 K by CuK alpha and synchrotron x-ray diffractions and by UV-Raman spectroscopy. It is shown that the in-plane compressive strain in SrTiO3 significantly increases the transition temperature between cubic and tetragonal-antiferrodistortive phases and furthermore precipitates a ferroelectric polar phase at lower temperatures. The strain-temperature phase diagram based on the measurements shows a cubic-to-antiferrodistortive transition temperature that is substantially higher than predicted within a phenomenological Landau-Ginsburg-Devonshire treatment. In contrast the transition to the ferroelectric polar phase is found to be close to the temperature predicted, with the consequence that there is no crossing of the two phase transitions for temperatures up to 600 K.