This study demonstrates that the compression rate adds a new perspective to phase diagrams of solids. A particular pressure increase rate may trigger unexpected solid-state transformations, producing otherwise inaccessible phases. Our test case is l-serine, characterized by a complex high-pressure behavior with three known polymorphs. However, the critical pressure of each transition, the ranges of coexistence of polymorphs, and the existence of an elusive fourth phase remained open questions, here analyzed and solved using synchrotron powder X-ray diffraction at high pressure, under controlled pressure increase rates. Two parallel paths exist, and the composition of the system depends on the pressure increase rate and the steps during the compression. A slow and continuous compression favors phase IV, whereas phase II can be observed only with a rapid and sharp compression. No direct interconversion occurs between these phases. Moreover, phase III originates only from phase II but never from phase IV. By controlling the strategy of pressure increase, we obtained a powder of phase IV that enabled solving its unknown structure, which resulted as a distorted superstructure of phase I with a tripled a-axis.