An off-duty NYPD sergeant was punched in the face and knocked to the ground by a biker on the Upper East Side after he asked the reckless motorcyclist to slow down, police said Wednesday.

The 52-year-old sergeant was walking his dog at East 90th Street and First Avenue around 7 p.m. Tuesday when he spotted a man whizzing around on a green motorcycle and asked him to go easy, authorities said. 

The two men began fighting — and the motorist repeatedly punched the off-duty cop in the face, causing him to fall to the ground, police said. 

The ruthless attacker kept pummeling the sergeant while he was on the ground, before riding off on the bike, heading east on East 90th Street, authorities said. 

The injured cop was taken to Weill Cornell Medical Center with a broken left ankle and a cut on his right ear and lip, police sources said. 

The NYPD released photos early Wednesday of the suspect, who was still on the loose, describing him as between 35 and 45 years old, 5-foot-8 and 200 pounds.

He was last seen wearing a gray hoodie, black pants, helmet and riding the green bike.