It's the first time she's given details of the terrifying experience, which led to her retreating from the spotlight.
"Rape is like living murder, you are alive, but dead," she wrote.
"All I can say is it took an extremely long time, sometimes feeling never ending, to reclaim the shattered pieces of me."
Duffy had the UK's best-selling album of 2008 and won three Brit Awards and a Grammy, but she virtually vanished after releasing her second album in 2010.
Her lengthy written account, in which she does not name her attacker, comes a month after she posted the first details on Instagram.
She wrote: "It was my birthday, I was drugged at a restaurant, I was drugged then for four weeks and travelled to a foreign country.
"I can't remember getting on the plane and came round in the back of a travelling vehicle. I was put into a hotel room and the perpetrator returned and raped me."
The star said she then "could have been disposed of by him". She contemplated running away but was afraid he would call the police.
"I do not know how I had the strength to endure those days, I did feel the presence of something that helped me stay alive."
'Life in danger'
She explained that she flew back to the UK with him but "knew my life was in immediate danger" because he made veiled threats to kill her.
"It didn't feel safe to go to the police. I felt if anything went wrong, I would be dead, and he would have killed me. I could not risk being mishandled or it being all over the news during my danger."
Afterwards, the singer was at high risk of suicide, she revealed.
"I would not see someone, a physical soul, for sometimes weeks and weeks and weeks at a time, remaining alone," she added. "I would take off my pyjamas and throw them in the fire and put on another set. My hair would get so knotted from not brushing it, as I grieved, I cut it all off.
"In hiding, in not talking, I was allowing the rape to become a companion."
She said she considered changing her name and appearance and disappearing altogether to live in another country.
"I thought the public disclosure of my story would utterly destroy my life, emotionally, while hiding my story was destroying my life so much more. I believe that not singing is killing me," she wrote.
"So, I just have to be strong and disclose it and face all my fears head on. I've come to realise I can't erase myself, I live in my being, so I have to be completely honest and have faith in the outcome."
She concludes: "I can now leave this decade behind. Where the past belongs. Hopefully no more 'what happened to Duffy questions', now you know … and I am free."