The victim, a 30-year-old man, had just entered the South Shore Hyundai dealership at 599 W. Merrick Rd. at 6:50 a.m. when he was approached by the suspect, who punched him in the face, police said.
The victim fell to the floor and the fall caused pain in his face, left arm and lower back, police said. The suspect then pulled a knife and demanded the victim's "personal property and keys," according to a news release.
The victim handed over his keys and his wallet, which contained credit cards, personal papers and an undetermined amount of cash, police said.
Before the suspect left, he damaged a desk and threw items off the counter, police said. He was last seen fleeing eastbound on Merrick Road.
Police had no additional information on the medical condition of the victim.
Anyone with information can call Crime Stoppers at 800-244-8477. All calls will be kept confidential.