An Qingxu
By this point, An Lushan had been assassinated and replaced as the emperor of Yan by his son An Qingxu. After An Qingxu took the throne, he sent the general Yin Ziqi () to attack Suiyang (i.e., the capital of Suiyang Commandery). Xu, who had returned to Suiyang by that point, sought aid from Zhang. Zhang therefore left his officer Lian Tan () in charge of defending Ningling and took most of his forces to Suiyang, defending the city together. They repelled Yin's attack initially, but Yin soon regrouped and put Suiyang under siege. Xu, citing the fact that he was a civilian official and not well versed in military matters, bestowed the command of their joint forces on Zhang and became only responsible for logistics, leaving the military matters to Zhang.