  Nematic states are characterized by rotational symmetry breaking without
translational ordering. Recently, nematic superconductivity, in which the
superconducting gap spontaneously lifts the rotational symmetry of the lattice,
has been discovered. However the pairing mechanism and the mechanism
determining the nematic orientation remain unresolved. A first step is to
demonstrate control of the nematicity, through application of an external
symmetry-breaking field, to determine the sign and strength of coupling to the
lattice. Here, we report for the first time control of the nematic orientation
of the superconductivity of Sr$_x$Bi$_2$Se$_3$, through externally-applied
uniaxial stress. The suppression of subdomains indicates that it is the
$\Delta_{4y}$ state that is most favoured under compression along the basal
Bi-Bi bonds. These results provide an inevitable step towards understanding the
microscopic origin of the unique topological nematic superconductivity.
