  We perform nuclear magnetic resonance (NMR) measurements of the oxygen-17
Knight shifts for Sr$_2$RuO$_4$, while subjected to uniaxial stress applied
along [100] direction. The resulting strain is associated with a strong
variation of the temperature and magnetic field dependence of the inferred
magnetic response. A quasi-particle description based on density-functional
theory calculations, supplemented by many-body renormalizations, is found to
reproduce our experimental results, and highlights the key role of a van-Hove
singularity. The Fermi liquid coherence scale is shown to be tunable by strain,
and driven to low values as the associated Lifshitz transition is approached.
