In 712, Emperor Ruizong yielded the throne to Li Longji, who took the throne as
Emperor Xuanzong. Li Sizhi's title was upgraded to Prince of Tan. In 715, however, he was bypassed for the crown prince position when his younger brother
Li Siqian the Prince of Ying was selected, as Li Siqian's mother Consort Zhao was then Emperor Xuanzong's favorite concubine. In 716, Li Sizhi was made the
Protectorate General to Pacify the West and received the title of "Comforter of the Non-Han of the Hedong [(modern
Shanxi)], Guannei [(i.e.,
Guanzhong)], and Longyou [(modern eastern
Gansu)] Regions" (安撫河東關內隴右諸蕃大使,
Anfu Hedong Guannei Longyou Zhufan Dashi), although there was no evidence that he actually left the capital
Chang'an. In 725, Li Sizhi's name was changed to Li Tan, and his title was changed to Prince of Qing. In 727, Li Tan became the military governor (
jiedushi) of Hexi Circuit (河西, modern central and western Gansu) as well as the commandant at Liang Prefecture (涼州, roughly modern
Wuwei, Gansu), but, as was the case with 10 other brothers of his who received similar titles, did not actually take up the command. In 736, Li Tan's name was changed to Li Cong. In 737, Li Siqian (the heir), whose name had been changed to Li Ying, was forced to commit suicide after false accusations of treason by Emperor Xuanzong's then-favorite concubine
Consort Wu, who wanted her son Li Mao () the Prince of Shou to be crown prince. However, Emperor Xuanzong subsequently selected another son,
Li Yu the Prince of Zhong, crown prince, bypassing Li Cong again in the process. Later when Chancellor
Li Linfu tried to depose the new Crown Prince Li Yu, he tried to persuade Emperor Xuanzong to create Li Cong as Li Cong was the eldest, but Emperor Xuanzong silently replied, "Cong, my eldest son, in early years when he hunted in the imperial garden, his face was hurt and badly injured." The emperor was implying that someone with a disfigured face was not fit to rule, and this might be the reason why Li Cong was always bypassed and never became crown prince. Though Li Linfu still insisted on deposing Li Yu, Emperor Xuanzong never replaced him, as Li Yu always behaved courteously. In 742, Li Cong became military governor of Hedong Circuit. == Posthumous honors ==