After out-pacing his team-mate Lewis Hamilton in FP2, Nico Rosberg reckons that the data they gathered during Friday’s practice sessions could be wasted if the weather takes a drastic turn.
During FP1, the Championship leader said that the track was very cool compared to the afternoon session, something that it is very common in Austin.
He also noted that there was a clear difference in performance between the two practice rounds.
"The running of the car was different between P1 and P2 because of the temperature; it was very cold when we ran this morning," said the German.
"The weather also had a bearing on the balance of the car [throughout practice] as it altered as a consequence of the changing wind direction around the track.
"The weather is going to change again, as it does frequently here in Austin, and it means what we learn today might not necessarily help us for Saturday."
Hamilton, who was pipped by Red Bull’s Daniel Ricciardo in FP2 as he split the two Mercedes', was delighted that the reliability of his Silver Arrow was good.
"It's been a great day for reliability and getting our programme done," said the Brit, who topped the times in the morning session.
"Given the wet weather conditions here last year and the lack of reference points for each tyre compound Nico and I had to cover as much ground as possible to help with our understanding.
"We've still got to go away and analyse the data overnight to make sure we're ready."
— Nico Rosberg (@nico_rosberg) October 21, 2016