The victim, the father of a family in his 40s, was attacked on Friday night at around 10pm outside Rooftop 42 on the Rue du Rhône, the 20 Minutes newspaper reported online.
One of the bouncers has been arrested for “attempted murder”, while an investigation is continuing, the newspaper said, citing information from the Geneva prosecutor's office.
“We were at the bottom of an elevator when the bouncers started to beat this man,” a witness told the daily.
“There were three bouncers on him,” said the witness, who had spent the night at the bar.
“They were punching him, accusing him of having lacked respect,” the witness added.
“Then one of them hit him so hard that he was propelled to the ground, hitting himself in the head and losing consciousness.”
The bouncers then left the injured man.
Witnesses called an ambulance, which transported him to hospital in serious condition, hovering between life and death, 20 Minutes said.
The victim's lawyer told the newspaper that video surveillance cameras recorded the incident “so we should soon know exactly what happened”.