Li Su
In 818, when the general Li Su served as the military governor of Shannan East Circuit (山南東道, headquartered in modern Xiangfan, Hubei), Zheng went to serve under Li Su, and as Li Su received medical services from Zheng, he became trusting of Zheng and commissioned Zheng as a guard commander. Later, when Li Su was moved to be the military governor of Wuning Circuit (武寧, headquartered in modern Xuzhou, Jiangsu), Zheng followed him, and became even more involved in the decisions. It was said that Zheng was treacherous but well-spoken, and was capable of detecting what people desired, and therefore, the suggestions he made Li Su were the ones that pleased Li Su. As a result, he became quite powerful at the headquarters, and the officers feared him. At that time, the eunuch Wang Shoucheng served as the monitor of Wuning, and Wang disliked Zheng immensely, and, on one occasion, Wang pointed out how the army was resentful of Zheng. Li Su stated to Wang, "Although it is true, Zheng is an amazing man. You, general, should meet him. If you disagree, then he can be removed." Li Su then had Zheng meet Wang. Wang initially did not want to and refused to seat Zheng, but once Zheng began talking with him, Wang was pleased with Zheng's analyses, and invited Zheng in to talk further. Thereafter, Zheng became a close associate of Wang's.