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Tian Xu was born in 764, during the reign of Emperor Daizong. At that time, his father Tian Chengsi was serving as the military governor ("Jiedushi") of Weibo Circuit and ruling it semi-independently from the imperial government. He was Tian Chengsi's sixth son, out of 11. As Tian Chengsi found his nephew (Tian Xu's cousin) Tian Yue to be more capable than his own sons, he had Tian Yue serve as his deputy and intended heir, having his sons assist Tian Yue. After Tian Chengsi died in 779, Tian Yue succeeded him as the military governor of Weibo Circuit with permission from Emperor Daizong, and Tian Xu thereafter served under Tian Yue.