Zhou Tian himself often chose to retreat when encountering enemies he couldn't defeat, so based on his experience, when he saw the changes in Pan Shi's expression, he could naturally guess what was going on in his mind quite accurately. Not to mention, given the situation at the time, it wasn't difficult for Zhou Tian to predict Pan Shi's next response.
Thus, although Pan Shi's actions seemed very sudden, Zhou Tian had actually anticipated them beforehand. In that situation, when he saw Pan Shi attempting to flee, Zhou Tian naturally wouldn't let him escape easily.
In fact, whether Zhou Tian was prepared was not the important matter. Given the conditions of both parties, as long as Zhou Tian didn't allow him to leave, Pan Shi's escape was doomed to be difficult.