High spin states in Sly up to the probable 33/2 + and 29/2-yrast states have been measured via the reaction 5SNi (28Si, c~p), using the Cologne tandem accelerator. Directional correlations of oriented nuclei were determined and the level scheme was extended with lifetimes measured by means of DSA and recoil distance techniques. Reduced transition strengths inferred from lifetimes were consistent with the results of Hartree-Fock-Bogolyubov calculations, which predict a nearly axially symmetric deformation of/72 ~ 0.37 for the n = + band. Predictions reveal an alignment driving the system to a smaller and triaxial deformation with f12 = 0.23 and 7=-30 ~ although the data show this crossing to be somewhat delayed. Particle-rotor calculations for the one-quasiparticle bands corroborate the predictions of deformation parameters based on the cranking model.