Sauber drivers Kamui Kobayashi and Sergio Perez felt their car fared worse than most in the very warm weather conditions during Friday's practice sessions for the Brazilian Grand Prix.
With track temperatures reaching a high of 42 degrees centigrade, tyre wear and overheating became a severe issue for both drivers.
"We had a busy day, but performance-wise we are not good yet. Because of high track temperatures the rear tyres overheated," said Kobayashi, whose learned earlier on Friday that he would be ousted in favour of Esteban Gutierrez next year.
"The laptimes were slower in the afternoon than the morning. The rear is really weak and so far I don't know how to cool the tyres. We will work on that."
McLaren-bound Perez shared his team-mate's view.
"The heat just doesn't suit us," he said. "I found the car difficult to drive this afternoon and I think some big changes are required. Let's see how we can improve."
Sauber's head of track engineering Giampaolo Dall'Ara backed up his drivers' concerns over lack of grip and pledged to endeavour to rectify the problem before qualifying.
"We have a long night ahead of us as we are struggling with the grip, but we have collected lots of data regarding downforce and set-up," said Dall'Ara.
"Now we just have to put it all together to understand where we can find performance and hopefully to be where we want to be tomorrow."