Jessica Simpson attempted to "fix" a heartbroken Nick Lachey months after their marriage separation by having sex with him one last time.

The singer filed for divorce from the 98 Degrees frontman after three years of marriage in 2005, and in her new memoir, Open Book, she reveals they didn't speak for months after their split.

But, after watching Lachey pour out his emotions over the failed marriage in an MTV documentary chronicling the making of his heartbreak album What's Left of Me, she invited him over to her house to talk.

"I just wanted to know what he thought of me now, because at that point I had no idea," she writes. "I watched him portray himself as a victim, casting me as this selfish person. I knew he did this to hurt me."

Simpson blamed herself for Lachey "breaking down in front of the world" and although she still had feelings of resentment towards her ex, she felt a need to "fix" him.

"I asked to meet him at my house...," she recalls. "He rang the bell, and out of reflex I hugged him. I meant it, too. Despite my anger, I missed him."

They listened to What's Left of Me together, and then ended up in bed.

"I was numb, just blank. How do you react when you find out you have apparently hurt someone so deeply that they feel entitled to such actions?" she shares. "I felt manipulated into some revenge fantasy, but I had put myself in this situation. I didn't know any other way to make it better, so I slept with him."

The intimate encounter took place shortly before the former couple's divorce was finalised in the summer of 2006, and for Simpson, it served as confirmation that their relationship was officially dead.

She continues, "He didn't stay the night. I was relieved because I could feel his hate. The whole situation was very dark.

"When he walked out the door, I knew I would never see him again."

Lachey went on to romance Vanessa Minnillo, the MTV personality who featured in the 2006 music video for What's Left of Me. They wed in 2011 and now share three children.

Meanwhile, Simpson famously dated John Mayer and former American footballer Tony Romo, before finding love with her now-husband, Eric Johnson, the father of her three kids.