During Emperor Suzong's reign
After Emperor Suzong took the throne, while he did not at that point make his oldest son the Prince of Guangping crown prince, he gave Li Chu the title of supreme commander of the armies, and he gave Li Jingzhong dual titles on Li Chu's staff -- serving as the head of the crown prince's household and acting assistant of military affairs to the supreme commander . Emperor Suzong entrusted Li Jingzhong with the important secrets, and Li Fuguo became in charge of receiving important reports, as well as distributing military command seals and signs. He also changed Li Jingzhong's name to Huguo . By this point, Li Huguo ate a vegetarian diet and often acted as Buddhist monks did; further, when he was taking a break, he would often hold prayer beads, and people believed at that point he was benevolent. Later, when Emperor Suzong, while still fighting Yan forces, moved his headquarters from Lingwu to Fengxiang in spring 757, he gave Li Huguo a slightly greater title as the head of the crown prince's household , and changed his name again to Fuguo .
After joint Tang and Huige forces recaptured Chang'an later in 757 under Li Chu's command, Emperor Suzong returned to Chang'an. He bestowed on Li Fuguo a number of titles that gave him responsibility over a number of financial affairs of the state. He also gave Li Fuguo the honorific title of ''Kaifu Yitong Sansi'' and created him the Duke of Cheng. The officials' reports continued to go through Li Fuguo, and Li Fuguo established a bureau with a number of agents with the responsibility of secretly finding out officials' faults. Even criminal cases were ruled on by Li Fuguo, and he was often making orders, in Emperor Suzong's name, reversing officials' decisions. No official dared to speak against him or even to refer to him by official title, instead addressing him as "Master Five," using a form of address from a servant to a master. Even the chancellor , who was from a highly honored household, paid him respect as a son or a nephew would, calling him, "Father Five." Emperor Suzong also gave Li Fuguo, despite his eunuch status, the grandnephew of the deceased official Yuan Xisheng as his wife, and promoted Lady Yuan's uncle.
After Emperor Xuanzong's return to Chang'an, he took residence at Xingqing Palace , which was converted from his residence as an imperial prince. Gao Lishi and the general Chen Xuanli attended to him, as did Emperor Xuanzong's younger sister Li Chiying the Princess Yuzhen, the lady in waiting Ru Xianyuan , and the eunuchs Wang Cheng'en and Wei Yue . These attendants of Emperor Xuanzong did not respect Li Fuguo. To retaliate, Li Fuguo began to try to convince Emperor Suzong that Emperor Xuanzong and his attendants were plotting to seize power back. In 760, with Emperor Suzong's tacit, although not explicit, approval, on one occasion when Emperor Xuanzong was out riding, Li Fuguo intercepted him and forced him to move back to the main palace. Even on that occasion, however, Gao would not submit to Li Fuguo, and even yelled at Li Fuguo to force him to get off his horse and to escort Emperor Xuanzong on foot, along with Gao. Soon after Emperor Xuanzong was forcibly moved, Li Fuguo forced Chen to retire, Li Chiying to return to her temple , and exiled Gao, Wang, Wei, and Ru. Emperor Suzong had his daughters Princesses Wan'an and Xianyi attend to Emperor Xuanzong, but Emperor Xuanzong, depressed over his forced movement and the exile of his attendants, began to be ill. Emperor Suzong regretted this and considered killing Li Fuguo, but feared the fact that Li Fuguo had command of the imperial guards, and therefore did not act.
Throughout the years, Empress Zhang and Li Fuguo's alliance held. However, as of spring 762, when both Emperors Xuanzong and Suzong were seriously ill, Empress Zhang and Li Fuguo had begun to be rivals. She summoned Li Chu and tried to persuade him to join her in killing Li Fuguo and his ally Cheng Yuanzhen. Li Yu declined, and she instead tried to persuade his younger brother Li Xi the Prince of Yue, to join her. Li Xi agreed. She and Li Xi thereafter had the eunuch Duan Hengjun selected some 200 strong eunuchs, ready to ambush Li Fuguo and Cheng. On May 14, Empress Zhang issued an order in Emperor Suzong's name, summoning Li Yu. Cheng found out and informed Li Fuguo, who intercepted Li Yu at the palace gate and then escorted him to the camp of the imperial guards under Li Fuguo's command. The guards under Li Fuguo's command then entered the palace and arrested Empress Zhang and Li Xi; the other eunuchs and fled, leaving Emperor Suzong without care. On May 16, Emperor Suzong died, and Li Fuguo thereafter executed Empress Zhang and Li Xi, as well as Li Xian the Prince of Yan, and then declared Li Yu emperor .