Holiday cheer turned into a nightmare when two workers hanging Christmas decorations in a cherry picker were hit by an 18-wheeler on the Upper West Side, authorities said.

The men had just finished hanging lights across Columbus Avenue near West 67th Street and were in the process of lowering themselves around 1:30 a.m. Tuesday, when a UPS tractor-trailer smashed into the bucket they were standing in, knocking the victims to the ground, officials said.

Both men suffered head trauma and were taken to New York-Presbyterian Hospital. One suffered potentially life-threatening injuries and the other man is listed in serious condition, authorities said.

The driver remained at the scene and was not expected to face criminal charges.