It took six months of dating other people before Justin Timberlake and Jessica Biel decided to make their early relationship exclusive.

The couple was first linked in January, 2007 and wed in Italy in 2012, but in his new memoir Hindsight: And All the Things I Can't See in Front of Me, Justin reveals it took them some time to build up their romance following their first meeting at a Hollywood party.

"When I met Jess, it was undeniable how beautiful she is," the singer recalls.

"I made some sort of sarcastic comment, really dry. Nobody got it except her. She laughed, and I noticed, all of a sudden, and in the way where you wonder if a person's like you, if they have a very dry, dark sense of humour, too."

Justin didn't work up the courage to ask Jessica for her number that night, but he made sure to ask her out the next time they crossed paths, when the Blade: Trinity star and her friends attended one of his concerts on the FutureSex/LoveSounds Tour.

"Then the Golden Globes came up, and that's when we planned on seeing each other," he shares.

However, they were both hesitant to treat the budding romance seriously until six months into their relationship, when they finally realised they had started to fall in love.

"We were both still seeing other people, keeping ourselves safe from getting hurt, from really putting ourselves out there," Justin explains. "It took a bit for both of us to admit to ourselves that we were really, really into each other."

They eventually decided to stop dating around and give their love a chance to blossom, and now Justin cannot imagine his life without Jessica, the mother of his three-year-old son Silas.

"We've had a lot of unforgettable times," he continues. "She's become a huge influence on my life, and I have such an admiration for her, especially seeing her as a mother now... She changed me. She changed my life."

Elsewhere in the new book, the Can't Stop the Feeling! hitmaker revisits the huge emotional impact a heartbreaking split from an unnamed ex had on him, leading Justin to write his 2002 break-up anthem Cry Me a River in no time at all.

"I've been scorned. I've been p**sed off," he tells fans. "I wrote Cry Me a River in two hours."

The track has long been rumoured to detail the end of his three-year romance with Britney Spears in 2002, and while Justin has never directly confirmed the speculation, he fuelled the gossip by using a blonde lookalike for the accompanying music video.

And Justin insists he "didn't plan" on writing the tune.

"The feelings I had were so strong that I had to write it, and I translated my feelings into a form where people could listen and, hopefully, relate to it," he adds. "People heard me and they understood it because we've all been there."