Sauber driver Adrian Sutil says that the C33 is still a difficult car to control.
Sutil was 13th quickest during practice for the Malaysian Grand Prix at Sepang.
"In comparison to Melbourne, we made a few steps forward," he said. "The things we had on our plan partly worked out. However, the car is difficult to control and it is not easy to drive. We have a lot work ahead of us. We made steps in the right direction and it is working better than in Melbourne."
Team-mate Esteban Gutierrez explained that the C33 feels better under braking than it did two weeks ago.
"I think it was a positive day," he said.
"We managed to have no problems during the entire day. From that point of view, it is positive. Besides that, we managed to do our planned programme with both cars, so we collected interesting information which we can use to make the right decisions for tomorrow.
"In comparison to Melbourne, we made a step forward as the braking stability is better. We still have further things to develop and to improve. We have to evaluate the high fuel and find the best compromise for the race."