  The single-layered ruthenate Sr$_2$RuO$_4$ is one of the most enigmatic
unconventional superconductors. While for many years it was thought to be the
best candidate for a chiral $p$-wave superconducting ground state, desirable
for topological quantum computations, recent experiments suggest a singlet
state, ruling out the original $p$-wave scenario. The superconductivity as well
as the properties of the multi-layered compounds of the ruthenate perovskites
are strongly influenced by a van Hove singularity in proximity of the Fermi
energy. Tiny structural distortions move the van Hove singularity across the
Fermi energy with dramatic consequences for the physical properties. Here, we
determine the electronic structure of the van Hove singularity in the surface
layer of Sr$_2$RuO$_4$ by quasiparticle interference imaging. We trace its
dispersion and demonstrate from a model calculation accounting for the full
vacuum overlap of the wave functions that its detection is facilitated through
the octahedral rotations in the surface layer.
