  We present the first observation of K- and \phi absorption within nuclear
matter by means of \pi- -induced reactions on C and W targets at an incident
beam momentum of 1.7 GeV/c studied with HADES at SIS18/GSI. The double ratio
(K-/K+)W / (K-/K+)C is found to be 0.319 \pm 0.009(stat)+0.014-0.012 (syst)
indicating a larger absorption of K- in heavier targets as compared to lighter
ones. The measured \phi/K- ratios in \pi-+C and \pi^- +W reactions within the
HADES acceptance are found to be equal to $0.55 \pm 0.03(stat)+0.06-0.07$
(syst) and to $0.63 \pm 0.05(stat)-0.11+0.11$ (syst), respectively. The similar
ratios measured in the two different reactions demonstrate for the first time
experimentally that the dynamics of the \phi meson in nuclear medium is
strongly coupled to the K- dynamics. The large difference in the \phi
production off C and W nuclei is discussed in terms of a strong \phiN in-medium
coupling.
