  We report on the first detection of a pre-supernova (SN) evolution in the
X-ray regime. The results are based on ROSAT observations of SN 1993J ranging
from six days to five years after the outburst. The X-ray observations are used
to probe the SN shell interaction with the ambient circumstellar matter (CSM).
After exploring various scenarios that might explain the observed X-ray
lightcurve with a t^-0.27 rate of decline, we present a coherent picture in
terms of the interaction of the SN shock front with the CSM deposited by the
progenitor's stellar wind. During the observed period, the SN shell has reached
a radius of 3x10^17 cm from the site of the explosion, corresponding to ~10^4
years in the progenitors stellar wind history. Our analysis shows that the
mass-loss rate of the progenitor has decreased constantly from \dot{M} =
4x10^-4 to 4x10^-5 M_sun/yr (v_w/10 km/s) during the late stage of the
evolution. Assuming a spherically symmetric expansion, the circumstellar matter
density profile is found to be significantly flatter (\rho_csm \propto r^-1.63)
than expected for a constant mass-loss rate and constant wind velocity profile
(r^-2). The observed evolution either reflects a decrease in the mass-loss
rate, an increase in the wind speed or a combination of both, indicating that
the progenitor likely was making a transition from the red to the blue
supergiant phase during the late stage of its evolution.
