In finite-state systems, true existential properties admit witnesses in form of lasso-shaped fair paths. When dealing with the infinite-state case (e.g. software non-termination, model checking of hybrid automata) this is no longer the case. In this paper, we propose a compositional approach for proving the existence of fair paths of infinite-state systems. First, we describe a formal approach to prove the existence of a non-empty under-approximation of the original system that only contains fair paths. Second, we define an automated procedure that, given a set of hints (in form of basic components), searches for a suitable composition proving the existence of a fair path. We experimentally evaluate the approach on examples taken from both software and hybrid systems, showing its wide applicability and expressiveness.