Help me find this anime.

I saw a brief clip but forgot to save it.

It was about a guy who took martial arts with a 2nd grade girl, but she was, for some unknown reason, crazy strong. She even caused her sensei to retire after beating him as a child.

Then they grow up and he runs into her again, and I guess his mission is to best her? IDK.

Anyone know it? I've been racking my brain and the internet for it for days.