  Real-world topological semimetals typically exhibit Dirac and Weyl nodes that
coexist with trivial Fermi pockets. This tends to mask the physics of the
relativistic quasiparticles. Using the example of ZrTe5, we show that strain
provides a powerful tool for in-situ tuning of the band structure such that all
trivial pockets are pushed far away from the Fermi energy, but only for a
certain range of Van der Waals gaps. Our results naturally reconcile
contradicting reports on the presence or absence of additional pockets in
ZrTe$_5$, and provide a clear map of where to find a pure three-dimensional
Dirac semimetallic phase in the structural parameter space of the material.
