  Sr$_x$Bi$_2$Se$_3$ and the related compounds Cu$_x$Bi$_2$Se$_3$ and
Nb$_x$Bi$_2$Se$_3$ have attracted considerable interest, as these materials may
be realizations of unconventional topological superconductors.
Superconductivity with T$_c$ ~ 3 K in Sr$_x$Bi$_2$Se$_3$ arises upon
intercalation of Sr into the layered topological insulator Bi$_2$Se$_3$. Here
we elucidate the anisotropy of the normal and superconducting state of
Sr$_{0.1}$Bi$_2$Se$_3$ with angular dependent magnetotransport and
thermodynamic measurements. High resolution x-ray diffraction studies underline
the high crystalline quality of the samples. We demonstrate that the normal
state electronic and magnetic properties of Sr$_{0.1}$Bi$_2$Se$_3$ are
isotropic in the basal plane while we observe a large two-fold in-plane
anisotropy of the upper critical field in the superconducting state. Our
results support the recently proposed odd-parity nematic state characterized by
a nodal gap of $E_u$ symmetry in Sr$_x$Bi$_2$Se$_3$.
