Donaldson defeated the defending UK champion Mark Selby, 6–3, in the last 64 at the 2017 UK Championship, but was knocked out in the next round by Li Hang. Three months later, he reached the semi-final stage of a ranking event for only the second time in his career, at the 2018 Gibraltar Open; he won five matches before a 3–4 semi-final defeat against eventual champion Ryan Day.