A numerical analysis of step-path failure mechanisms in rock slopes is provided based upon simulations performed using a discrete element method specifically enhanced for the modeling of jointed rock masses. Fracturing of the intact rock as well as yielding within discontinuities can be simulated to determine the failure surface without any a priori assumption on its location. For both coplanar and non-coplanar sets of discontinuities, failure is the result of the propagation of tensile microcracks that develop in the rock bridges from the tips of pre-existing discontinuity planes in a way similar to wing cracks extensions that can eventually coalesce to form extended step-path failure surfaces. Sensitivity analyses are performed to better understand the critical mechanisms that lead to slope failure and to discriminate between the respective roles played by intact rock and planes of weakness at the onset of failure. For a randomly distributed set of joints that share the same preferential orientation, failure is shown to be dependent on the frictional strength mobilized on the joint surfaces. The results confirm the critical need for a comprehensive and extensive characterization of both mechanical and geometrical properties of discontinuities when assessing the stability of a rock mass.