Williams drivers Rubens Barrichello and Nico Hulkenberg believe they have work to do before the car is where they want it for qualifying in Brazil tomorrow.
Although Barrichello was seventh in morning practice at Interlagos, the duo were otherwise outside the top 10, and Hulkenberg suspects he will be hard-pressed to reach Q3 in qualifying.
"It looks very competitive out there this weekend so I think it could be quite a difficult task to get into the top 10 tomorrow," said Hulkenberg. "Personally, I didn't get a very clear run on the option on low fuel today but everything else went well. We still have to improve the car ahead of qualifying so we have work to do."
Barrichello agreed that while there was nothing major wrong with Williams's setup, improvements were required tonight.
"Today went OK," said the local driver. "We worked through everything we needed to and tried out various combinations. We need to make some improvements and get everything working for tomorrow as I don't think the car was any faster in the afternoon's session.
"Typically for Brazil, it's looking like it could be wet tomorrow, so we have to prepare for that."