Williams drivers Valtteri Bottas and Felipe Massa played down their comparative lack of pace during Friday's practice sessions for the Belgian Grand Prix.
Bottas ended the day in 14th position in the FW37, two places ahead of Massa, while Bottas only marginally crept into the top 10 during the opening session on Friday morning.
"Today our main focus was on aero testing, including the rear wing configuration and we also looked at the tyres as you would expect from a Friday," said Bottas, who finished on the podium in Belgium last year.
"We collected plenty of data throughout the day so we know that today’s times are not too representative, and we will continue to strengthen our performance throughout the weekend."
Massa says that the red flags - for Nico Rosberg's tyre failure and Marcus Ericsson's crash at Pouhon - clouded the picture for the squad.
"It was a typical Friday for Williams, we don’t go out trying to top the time sheets," he said.
"We completed a lot of the work we had set ourselves, only held back by the red flags which were unexpected.
"This has meant my long run pace isn’t as clear as I would have hoped, but we should be able to be competitive tomorrow and build as the weekend progresses."