A novel least-squares fitting procedure is presented that allows to retrieve strain profiles in ion implanted single crystals using high-resolution X-ray diffraction (XRD). The model is based on the dynamical theory of diffraction including a B-spline-based description of the lattice strain. The fitting procedure relies on the generalized simulated annealing algorithm which, contrarily to most common least-squares fitting-based methods, allows to find the global minimum of the error function (the difference between the experimental and the calculated curves) while being extremely fast. It is shown that convergence can be achieved in few hundreds of Monte-Carlo steps, i.e. a few seconds. The method is model-independent and allows to determine the strain profile even without any 'guess' regarding its shape. This procedure is applied to the determination of strain profiles in Cs-implanted yttria-stabilized zirconia (YSZ). The strain and damage profiles of YSZ single crystals implanted at different ion fluences are analyzed and discussed.