A Bronx man fatally clobbered his neighbor who was riding in an elevator of their apartment building the moment the doors opened, authorities said Friday.

The 40-year-old killer was shadow-boxing in the 7th floor hallway of the East 156th building where he lives at around 5 p.m. Thursday just before the beating, cops said.

When the elevator doors opened on the 7th floor, the attacker stepped into the elevator and forced 9th floor resident Bakary Darboe, 46, into the hallway of the Melrose building where he beat him senseless, cops said.

The assailant may have also used some sort of object to beat Darboe during the attack, which was captured on surveillance footage.

A neighbor witnessed the beginning of the assault and quickly shut the door.

The resident told police that the victim could be heard screaming, “Get off me!” police sources said.

After killing the man, the suspect fled the building but was nabbed a few blocks away by police who found him in bloodied clothes, cops said.

When police questioned the man – who has a lengthy rap sheet with 20 prior arrests dating back to the 1990s on charges including drug possession, robbery and reckless endangerment – he acknowledged that he got in a fight in the building, sources said.

The suspect quickly lawyered up and is currently being questioned by investigators at the 40th Precinct station house.

Charges against him are pending.

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