Claire Williams says the Williams team's current situation has nothing to do with its drivers.

Robert Kubica returned to F1 this year amid much fanfare, while his teammate is the reigning Formula 2 champion George Russell.

But Williams is flailing at the very back of the grid, and Pole Kubica - who has struggled the most - even says his car is different to that of his teammate.

"We have carried out several analyses of Robert's car, which according to Robert is different. George does not feel it as strongly as Robert and of course I don't know it myself, because I am not a racing driver," team boss Claire Williams told Brazil's UOL Sport.

"But we have to understand it. We are trying as hard as possible to give our drivers the sense and confidence that they have the same equipment," she said.

But she at least does not blame either Kubica or Russell for Williams' predicament.

"Given the car we gave them, I think it has been an extremely difficult period for them, but they both gave their best. That's all we can ask for in this situation," said Williams.

The team is hoping the worst of the struggles is now behind it. In Barcelona this weekend, Williams has prepared upgrades for the 2019 car.

"We hope they will be positive steps, to try and turn our season around," said Russell.