At any rate, he was one of the most powerful nobles under Dušan, and exercised considerable influence over him, as evidenced in the negotiations in July 1342 which led to the decision to support John VI Kantakouzenos in the Byzantine civil war of 1341–1347, in exchange for which he hoped to marry his daughter to Manuel Kantakouzenos. He was also active in the 1334 war with the Byzantine Empire, and was present during the subsequent peace negotiations together with Vratko Nemanjić, when Jovan was probably named "despotes" by Andronikos III Palaiologos. With the death of Hrelja in late 1342, when the latter's domain was split between Jovan Oliver and Dušan, he was able to further expand his lands, including the important towns of Štip and Strumica.