The incident, which was captured on surveillance video, occurred after 2 a.m. at the 630 North Franklin condos on Chicago's North Side.
The guard, Zoa Stigler, sustained a broken nose and black eye and may now need surgery. She was knocked off her feet when she was trying to help a man who appeared to be passed out on the sidewalk in front of the building.
"I am surprised I didn't fall down because the impact was really hard," Stigler said.
When she saw the man on the ground, she went out to the man, who appeared to be drunk. He said he was OK and then she told him that he needs to get up.
He didn't get up and minutes later, the man stood up and - surrounded by friends - attacks.
"He didn't say one word. He just walked up to me and threw the bottled water and hit me in my face," Stigler said.
Then he and his friends casually walked away, like nothing ever happened.
"You really need to be ashamed of yourself. You should never hit anyone, not because of I am a female but because I am a human being. You shouldn't hit a person doing their job," Stigler said.
Asad Khan, president of the condo board, hopes surveillance video helps Chicago police catch the man.
"Pick on someone your own size. If you can't handle your liquor then don't drink," Khan said.
Stigler, who is a mother and a new grandmother, was out of the hospital on Sunday, but still hopes police catch the man.
"You don't know how much damage you have done to me. I pray that they find you," said Stigler, who also is a student working the security guard job for extra cash.
"My reaction was, 'How could this happen? What would make a person do this?'" asked Sierra Vaughn, Stigler's daughter.