Later that day, when Li Shimin was examining the battlefield, Zheng forces suddenly arrived and surrounded Li Shimin. The Zheng general Shan Xiongxin (單雄信) directly attacked Li Shimin himself, and almost killing Li, Yuchi arrived and hit Shan with a spear, knocking Shan off his horse. Yuchi then escorted Li Shimin out of danger. Tang forces commanded by Qutu then counterattacked as well, defeating Zheng forces. Li Shimin awarded him a box of gold and silver and told him, "How does your repayment come so quickly!" Thereafter, he trusted Yuchi further. Meanwhile, Li Shimin's younger brother Li Yuanji the Prince of Qi, who was himself a ferocious warrior, heard that Yuchi had the particular skill of disarming opponents' lances, and requested a combat demonstration with Yuchi. Li Shimin ordered that they remove the sharp edges from their lances, but Yuchi responded, "I will obey the order and do so, but the Prince does not need to." Once they were into combat, Yuchi disarmed Li Yuanji three times. Li Yuanji was impressed but also angry from being humiliated. Meanwhile, Li Shimin selected some 1,000 elite soldiers, clad in black uniform and black armor, commanded by Li Shimin himself, to serve as the forward advance corps, with Yuchi, Qin Shubao, Cheng Zhijie, and Zhai Zhangsun (翟長孫) as his assistants. This corps subsequently accomplished much during the campaign against Wang and Dou Jiande the Prince of Xia, who came to Wang's aid. In engagement with Xia forward troops in spring 621, Yuchi was serving directly next to Li Shimin himself, and after they defeated Dou's forward troops, Li Shimin wrote to Dou, trying to dissuade him from aiding Zheng, but Dou did not relent.