Tian Jun
The first instance where Xu was recorded to have distinguished himself in battle was in 903, when Yang was facing rebellions by his subordinates Tian Jun the military governor of Ningguo Circuit (寧國, headquartered at Xuan Prefecture) and An Renyi () the military prefect of Run Prefecture (潤州, in modern Zhenjiang, Jiangsu). Yang initially sent Wang Maozhang to attack An at Run Prefecture, but Wang could not defeat An. He sent Xu to reinforce Wang's army, and Xu had his soldiers change into identical uniforms as Wang's. An, not knowing that reinforcements had arrived, had no reservations about reengaging Wang's army, and was defeated by Xu. (However, Run Prefecture did not fall at this time.) However, Xu's role became even more prominent when, later in the year, Yang received word that his brother-in-law Zhu Yanshou (the brother of his wife Lady Zhu) had agreed to join Tian's and An's rebellion and was set to rebel at Shou Prefecture (壽州, in modern Lu'an, Anhui). Xu, advised by his guest Yan Keqiu, submitted a proposal to Yang to trick Zhu Yanshou by having Yang pretend to be blind, even before Lady Zhu, and then issuing an order to summon Zhu Yanshou back from Shou Prefecture under the pretense of entrusting the affairs of the circuit to him. Zhu Yanshou believed this to be true and returned to Huainan's capital Yang Prefecture (), where he was killed in an ambush that Yang had Xu lay for him. After Zhu's death, Yang made Xu one of his two guard commanders.