After Nikephoros's assassination by John Tzimiskes in December 969, Peter, despite his close association with the murdered emperor, continued in active service during Tzimiskes's reign (r. 969–976), when he participated in the war against the Rus' in Bulgaria as leader of the "tagmata" of Macedonia and Thrace. He is mentioned as guarding the western wall of Dorostolon during its siege by the Byzantines, while Bardas Skleros guarded the eastern one. It is possibly during this campaign that Leo the Deacon's incident with his single combat against the "Scythian" leader took place.