  Precision studies of the properties of the top quark represent a cornerstone
of the LHC physics program. In this contribution we focus on the production of
$t\bar{t}$ pairs in association with one hard jet and in particular on its
connection with precision measurements of the top quark mass at the LHC. We
report a summary of a full calculation of the process $pp \to
e^+\nu_e\mu^-\bar{\nu}_\mu b \bar{b}j$ at NLO QCD accuracy, which describes
$t\bar{t}j$ production with leptonic decays beyond the Narrow Width
Approximation (NWA), and discuss the impact of the off-shell effects through
comparisons with NWA. Finally we explore the sensitivity of $t\bar{t}j$ in the
context of top-quark mass extraction with the template method, considering two
benchmark observables as case studies.
