Felipe Nasr admits he wanted to stay at Williams but couldn't pass up the opportunity of a race seat with Sauber in 2015.

Having been named as test and reserve driver at Williams, Nasr took part in a number of FP1 sessions during 2014 but saw Felipe Massa and Valtteri Bottas officially retained by the team at the Italian Grand Prix. While combining GP2 with his Friday practice running, Nasr was keen to step up in to an F1 race seat and was confirmed as a Sauber driver ahead of the Brazilian Grand Prix.

While happy with the opportunity to race in 2015, Nasr said his preference had been to continue his partnership with Williams.

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"Of course I wanted to stay here - but they have their commitment with Felipe and Valtteri so I had to move on," Nasr told the official F1 website. "I could not lose the momentum - I had to make the next step. Here things change very quickly so you better also move fast. One year of full racing is far better than a test role, sitting mostly at the sidelines."

And Nasr admits it will be a tough debut season with Sauber next year as the team looks to recover from a disappointing 2014 in which it failed to score a point.

"That depends a lot on the team - how the team is going to be next year. They had a difficult season so I think we have a lot of work to do - and of course I want to deliver. I know that so many young drivers are craving to make that step, so you better make the best of it - and enjoy the moment."