When does Grendel realize that Beowulf is no mere man?

Grendel plagued Heorot and Hrothgar’s people until Beowulf heard of the monster and decided to face it. Grendel mercilessly attacked Hrothgar’s people when they gathered in Heorot, forcing the people to abandon the mead hall built by their King. Beowulf, a valiant warrior from the Geats, traveled to the land of the Danes to fight the monster and end the peoples’ woes.
On arrival, Beowulf and his team of warriors were ushered to meet the King by a sentry stationed at the sea entry point. King Hrothgar welcomed them warmly and later discussed their visit. Beowulf informed the King that he had come to fight the beast. Beowulf and his band of warriors stayed in Heorot and waited for Grendel.
Grendel made his way to Heorot to perform his evil deeds. The monster entered the mead hall boldly, disregarding the warriors sleeping inside. Grendel attacked one of the warriors and devoured him as Beowulf watched. The beast grasped Beowulf, and the warrior grasped him back. At this point, Grendel realized that the warrior he was attacking was different. The beast realized that Beowulf was stronger than any man he had encountered before. Grendel panicked and attempted to flee, but it was too late.