Not through impotency, but in design, the angel suffered himself to be conquered, to encourage Jacob's faith and hope against the approaching danger. The hollow of his thigh, the joint of his hip-bone, or rather the hollow in which that joint was. The hollow of Jacob's thigh was out of joint, which was done that Jacob might see that it was not his own strength, but only God's grace, which got him this victory, and could give him the deliverance which he hoped for.
Related Commentaries of Genesis 32:25
Genesis 32:25
25 And when he saw that he prevailed not against him, he touched the hollow of his thigh; and the hollow of Jacob's thigh was out of joint, as he wrestled with him.