The student who exposed the serial rapist Reynhard Sinaga has said he feared he had killed him after he woke and fought back when he found he was being abused.

Sinaga, 36, was jailed this month for a minimum of 30 years for 136 rapes against dozens of young men in Manchester. Police believe he would have carried on offending had one of his victims not woken up during an attack and called 999.

This victim, known as Peter, has now told how he woke in a drunken state to find Sinaga on top of him on 2 June 2017.

Peter, who was 18 at the time, had been plied with a date rape drug and was half-naked when he woke midway through the assault on him. He described how he fought for his life and beat Sinaga until he lay motionless on the floor.

In an interview with the Sunday Mirror, he recalled: “I woke up disorientated, face down on a pillow. There was a quilt on the floor. My pants and jeans were down to the top of my knees; he was on top of me with his pants down to his ankles.”

The two men began to fight as Sinaga accused Peter of being an intruder. “He started shouting ‘Intruder! Intruder!’ I said: ‘Mate, calm down,’ then he headbutts me on my nose and starts biting me. I was so confused. We were grappling. He pulled me over on to the bed and bit my shoulder,” Peter said.

“He was a scrawny little guy. I was defending myself. I was stronger, I hit him a few times and he dropped to the floor. I was feeling scared. I knew I was in Manchester but that was about it.”

He added: “I thought I might have killed him, I didn’t really know what I was doing, I was using my instincts and adrenaline to survive.”

After escaping Sinaga’s flat, Peter called his mother and asked her to pick him up. Initially Peter was arrested on suspicion of assaulting Sinaga.

He spent 11 hours in police cell before detectives discovered that Sinaga had filmed attacks on dozens of unconscious men on his iPhone, including the assault on Peter.

Sinaga had been posing as a good Samaritan outside clubs in central Manchester, inviting men back to his flat for a drink or to charge their phones before slipping the date rape drug gamma-hydroxybutyrate (GHB) into their drink. He then recorded himself raping them, sometimes for hours at a time.

Peter described feeling like an “accidental hero” as Sinaga’s crimes may not have been detected had he not accidentally left his phone in Peter’s pocket.

Peter said: “The police and close friends say I’m a hero for stopping the biggest ever rapist. I feel like the accidental hero. If I hadn’t woken he’d still be doing it now.”

He said police at first did not believe he had been raped and he was not examined until two days later.

He told the Sunday Mirror: “They took my fingerprints and DNA. They questioned me like an offender, not a victim. He [Sinaga] was in hospital and they told me they would question him when he was released.

“I repeatedly said ‘I think he’s tried to rape me’ but they didn’t do any tests. That’s the one thing I thought wasn’t right at the time.”

After Peter handed them Sinaga’s phone, detectives found hundreds of clips of men being abused, and Sinaga was arrested. Peter went on to become a key witness in Sinaga’s trials.

Greater Manchester police said they had learned lessons from the case after admitting there were delays in treating Peter as rape victim.

A spokesman said: “We accept samples were not taken expediently in this case and we have since taken steps to strengthen our procedures going forward. Learnings have been cascaded to detectives and will also be incorporated into future policy.”