The top-quark mass is a fundamental parameter of the Standard Model and its determination is of great relevance per-se and for new physics scenarios. The demand for further improvements in the precision and accuracy with which the top-quark mass is determined brings new challenges and makes clear the importance of combining single measurements and the need for an accurate top-quark mass definition. The recent results from the ATLAS collaboration presented in this talk address such challenges by measuring the top-quark mass in an unusual final state identified by the presence of a low-energy muon from $t \bar t$ semileptonic decay, as well as measuring the top-quark mass with alternative theoretical definitions.