A bar owner who was drunk flew into a rage and became violent when he was turned away from a neighbouring bistro that had closed for the night.

Unhappy that his party was not allowed to enter Z Bistro and Bar in Prinsep Street, Raymond Chiang Zhi Hao, 30, urinated at its entrance and hit its glass door thrice with a metal stand.

Chiang and one of his employees, Kenneth Yang Kelun, 33, also assaulted the bistro owner, Mr Marcus Lew Mui Soon, 43.

Yesterday, the duo pleaded guilty to one count of causing hurt. Chiang was sentenced to 13 weeks' jail and Yang, three months.

Deputy Public Prosecutor Sruthi Boppana told the court that Chiang, who runs Life of Liquor near Z Bistro, was not on good terms with Mr Lew even before the incident.

"Raymond and his employees... had previously placed a cage for burning items during the (Chinese) seventh month in August last year next to the victim's bar, causing the burnt ashes to enter the bar," she said.

"Despite the victim having spoken to them to not act in such a manner, Raymond and his employees had repeated the same acts, prompting the victim to call the police, to Raymond's displeasure."

The court heard that Chiang and his friends had gone to Z Bistro at 4.40am on March 19.

Chiang created a ruckus and became violent when they were turned away.

He argued with Mr Lew and shoved the older man's head against a wall. Chiang also slapped him five times.

Mr Lew did not retaliate.

Yang then joined Chiang in the attack, with both punching Mr Lew repeatedly.

Mr Lew managed to get away from his attackers and ran up to the second storey of his bar, where he hid behind a counter. The duo went after him and continued to hit him.

The assault went on for six minutes until Mr Lew's employees and patrons intervened.

Chiang and Yang fled before police arrived at the scene.

Mr Lew was taken to Tan Tock Seng Hospital, where he was treated for injuries including multiple bruises and a fracture to his left eye socket. He was given five days of hospitalisation leave.

For causing hurt, Chiang and Yang could have been jailed for up to two years and fined up to $5,000.