Robert Kubica looks set to add Le Mans to his race programme in 2018.
The Pole was scheduled to test the Williams in Barcelona on Friday, but he gave up his running for race driver Sergey Sirotkin.
"It doesn't matter if the team asked me or I voluntarily gave priority to the regular drivers," he said. "But it was my suggestion because the weather was warm and it's the regular drivers who have to score the points, not me."
Kubica, 33, will be a vital addition to the British team in 2018, as Williams is struggling and the team's drivers are the young and inexperienced Sirotkin and Lance Stroll.
"Thursday morning was enough for me to understand the car to the point that I can describe its strengths and weaknesses to the engineers," he said.
Asked what the problems are, Kubica answered: "That's something that will stay in the engineers room.
"But I have a clear idea of where to improve the balance and the feel of the car. There are still issues with the behaviour of the car and the sensitivity of the car to the tyres.
"We have to make the tyre window larger in order to give Lance and Sergey confidence. It's important because they're still quite inexperienced."
While he sat out Friday's running, Kubica was later spotted meeting with former F1 boss Graeme Lowdon, who now runs the Manor sports car team at Le Mans, and it is expected that Kubica will sign a 2018 race deal.