Robert Kubica has admitted his comeback will only be defined by how much Williams have managed to improve their car for next season.
Williams had a terrible season in 2018 - finishing rock-bottom with just eight points; with by far the worst car on the grid.
Kubica is returning to Formula One after a severe injury he sustained back in 2011, crashing a rally-car and damaging his right hand as a result.
The 34-year-old is making one of the most sensational comebacks the sport has ever seen, but he believes his performance for next season will depend on what the engineers can do with the machine they produce.
"It's a difficult task and a very ambitious goal, but I think I'm well known for not taking on things that are light and easy," Kubica told Przeglad Sportowy newspaper.
"Whether it works or how close I am, much will depend on how I feel in the car and what the new car is like. How will it behave? This can greatly simplify or complicate the matter."
Sergey Sirotkin and Lance Stroll were the drivers for the British team last season, having to endure the aero inefficiencies and weaknesses of last season's FW41, and after using it during testing; Kubica described it as "dancing with a partner who cannot dance".
"If a car does not perform as the driver expects, it is not even so much about the speed and the lap times, but the feeling that it gives that can hinder the driver," Kubica concluded.