Pastor Maldonado says he is "happy" to be leaving Williams as he has wanted to part ways with the team for a while already.
Earlier this week, Williams confirmed that Felipe Massa would be joining them next year, replacing Maldonado as Valtteri Bottas' new team-mate.
The decision did not come as a shock as Maldonado's dissatisfaction with Williams had long been known.
"I am very happy," he told Autosport.
"Since a couple of months I was in conversation with the team and I wanted to leave the team. That is it. I am very happy for that.
"It was my decision. In Singapore I decide to leave the team more or less and it was 100 percent my decision."
Asked about why he wanted to leave the team that he first joined back in 2011, the 2012 Spanish GP winner stated that he believed he had done more for Williams than they had for him.
"I think I delivered more to the team than they for me.
"We find a victory, we find a very good result last year, and even this year we have been doing quite well races. But this is not everything.
"I am waiting for something more, I am expecting something more from F1."
He added: "Maybe I am learning more this year than last year.
"For sure it was a tough year, very hard for all the team, and my decision was because I was not good [happy] enough in the team personally.
"I wanted to try something different, and maybe it is a very important time for me and my career. It is a tough decision but a great one."
For now, though, Maldonado's future has yet to be secured.
The 28-year-old has been linked to Lotus but only if the team fails to secure the sponsorship they need for next season otherwise Maldonado and his PDVSA backing could be off to Sauber or Force India.
"For sure we are talking with Lotus, but I have a couple more options," he said.
"Maybe I have an idea [of what I want to do].
"We are working on that. I hope to have the final decision soon - and a clear route for next year."