A deranged knife-wielding attacker fatally stabbed a 69-year-old man in the back inside a Chinatown apartment, police said Sunday.

Feizhon Zheng, 50, was busted for murder for the stabbing, which unfolded at 7 Eldridge St. around 3:50 p.m. on Saturday.

It’s unclear how Zheng knew the victim, whose identity hasn’t been released pending family notification.

Zheng lives on Monroe street, a couple blocks away from the scene of the crime.