Nico Rosberg was pleased with the performance of the updates introduced by his Williams team for this weekend's Turkish Grand Prix.
"I think that the new aero parts we've brought out here are working as we expected them to, which is good," said Rosberg, who topped the times in the morning session.
"Having said that, levels of progress are always calculated in relation to other people and we are never sure where the others are, especially on a Friday.
"Today's sessions were mainly about tire testing. We covered a lot of ground with them, particularly in the afternoon session, so we now have a better understanding of how both the option and prime are working, but we will have another look at them tomorrow morning to figure out how to get the most out of them.
"It's clear that tire choice will be an interesting call for both qualifying and the race," added Rosberg, seventh quickest in the afternoon.
Teammate Kazuki Nakajima admitted he had endured an "strange" practice, having had problems to make his tires work properly. Despite that, the Japanese wound up fourth.
"It was quite a strange practice day, especially the afternoon's session when we tried the prime and option tyre compares," he said. "The tires really weren't working well today. I think our car is fairly quick, but we need to wait and check the data to see how we can get the best out of the tires."