"Returning to Formula 1 will not be easy," the 34-year-old told Polish broadcaster Telewizja Polska. "One of the biggest and most difficult challenges in my career is in front of me. But that's what I was striving for."
Not only that, the former BMW and Renault driver will be in a Williams, which last year was the slowest car in the field.
"The previous season was not easy for Williams," Kubica admitted. "But the regulations are changing and it may allow us to return to competing. It's not like we will suddenly be the best team, but we'll work hard to make the car better than the last one."
Kubica said he is not setting a 'concrete goal' in terms of actual qualifying or race results for 2019.
"My goal is maybe even more difficult than coming back to Formula 1," he said. "It's staying in Formula 1. Competition is great. The 20 best drivers in the world are here so it will not be an easy task, especially after such a long break."
"The car will be very important. If we manage to improve it, the goal will be easier."
