  The binary ruthenate, RuO$_2$, has been the subject of intense interest due
to its itinerant antiferromagnetism and strain-induced superconductivity. The
strain mechanism and its effect on the microscopic electronic states leading to
the normal and superconducting state, however, remain undisclosed. Here, we
investigate highly-strained epitaxial (110) RuO$_2$ films using
polarization-dependent oxygen K-edge X-ray absorption spectroscopy (XAS).
Through the detection of pre-edge peaks, arising from O:$2p$ - Ru:$4d$
hybridization, we uncover the effects of epitaxial strain on the
orbital/electronic structure near the Fermi level. Our data show robust
strain-induced shifts of orbital levels and a reduction of hybridization
strength. Furthermore, we reveal a pronounced in-plane anisotropy of the
electronic structure along the $[110]/[1\bar{1}0]$ directions naturally
stemming from the symmetry-breaking epitaxial strain of the substrate. The
$B_{2g}$ symmetry component of the epitaxially-enforced strain breaks a
sublattice degeneracy, resulting in an increase of the density of states at the
Fermi level ($E_F$), possibly paving the way to superconductivity. These
results underscore the importance of the effective reduction from tetragonal to
orthorhombic lattice symmetry in (110) RuO$_2$ films and its relevance towards
the superconducting and magnetic properties.
