Police say the suspect followed the 53-year-old victim and slashed him in the neck in front of 258 Schermerhorn Street just before 11 a.m.
Authorities say the man was approached from behind and then attacked.
He went into Chelsea Piers Fitness for help where employees called 911.
The victim was taken to NewYork-Presbyterian Brooklyn Methodist Hospital where he is expected to survive.
The suspect was last seen wearing a dark shirt, white shorts, and sneakers. He fled on foot towards Bond St.
There are no arrests, and the investigation is ongoing.
A spokesperson with Chelsea Piers Fitness released the following statement:
"This morning, an incident occurred on Schermerhorn street that did not involve a Chelsea Piers Fitness staff or club member. The injured individual entered Chelsea Piers Fitness after the incident, seeking assistance from one of our staff members who immediately called 911. The individual remained safely inside of our club until the police arrived. The police are investigating the incident and we are fully cooperative."
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