The sudden assassination attempt indeed caught the Tyrannosaurus off guard, and without any preparation, it was quite difficult for the beast to easily block Zhou Tian's killing move.
However, although the Tyrannosaurus was taken by surprise by Zhou Tian's assassination attempt at that moment, it was not an easy target for Zhou Tian to kill right then and there.
As a result, just as Zhou Tian's attack was about to hit the Tyrannosaurus, and when the beast had no ability to dodge the assassination move, it seemed like it had to endure Zhou Tian's strike. In reality, the Tyrannosaurus had managed in a very short time to move its vital points out of the way.