  We present the computation of the double real and real-virtual contributions
to top-antitop pair production in the quark-antiquark channel at leading
colour. The $q \bar q \to t \bar{t} g$ amplitudes contributing to the
real-virtual part are computed with OpenLoops, and their numerical stability in
the soft and collinear regions is found to be sufficiently high to perform a
realistic NNLO calculation in double precision. The subtraction terms required
at real-real and real-virtual levels are constructed within the antenna
subtraction formalism extended to deal with the presence of coloured massive
final state particles. We show that those subtraction terms approximate the
real-real and real-virtual matrix elements in all their singular limits.
