A woman suffered multiple facial fractures after a teenager out on bail for drug-related offences attacked and robbed her at an HDB lift lobby.

The 53-year-old woman was so shaken by the attack last October that she has undergone two psychotherapy sessions.

Her attacker, Rishi Christopher, who turned 20 last December, pleaded guilty yesterday to committing robbery with hurt. He also admitted to one count each of theft, criminal breach of trust and drug possession and consumption.

A gag order has been imposed to protect the woman's identity.

Police first arrested Rishi during a spot check in Toa Payoh on Sept 20 last year and his urine sample was later found to contain traces of a synthetic cannabinoid.

A brown substance in his possession contained a similar drug.

He was later released on bail - only to commit further offences.

On Oct 18 last year, Rishi stole items of clothing, including a T-shirt and a pair of shoes, worth more than $120 from a shop at Waterway Point mall in Punggol.

About three hours later, he spotted a woman walking past a block of flats in Punggol Way. He followed her into a lift, then got out with her on a higher floor, where he punched her face, causing her to drop her keys and mobile phone.

Deputy Public Prosecutor Rebecca Wong told District Judge Eddy Tham: "Before the victim could react, the accused grabbed (her) neck, choking her, and rained multiple punches on her face.

"The victim fell to her knees screaming in pain. The accused continued to punch (her) while pulling on her handbag and telling her to 'let go'. The victim refused."

The woman then grabbed hold of Rishi's groin and squeezed it hard, forcing him to release his grip. She also tried to hit him with her umbrella but he flung it aside.

After a struggle, Rishi grabbed the woman's keys off the floor and ran away. A resident who heard the commotion alerted police. Officers found the injured woman slumped against a wall at the lift lobby.

She was warded at Sengkang General Hospital until Oct 23 and given 27 days of medical leave.

The court heard that the police arrested Rishi in Yishun about four hours after the robbery.

DPP Wong also said that Rishi misappropriated an $850 e-scooter from a Yishun shop on Oct 17 last year after telling the store owner that he wanted to take it out for a test ride.

Rishi is expected to be sentenced on April 30.