Valtteri Bottas says Williams has a "big challenge" on its hands if the team is going to beat Force India at this weekend's United States Grand Prix.
The Finn was sixth quickest and just 0.1s behind Nico Hulkenberg in the opening practice session, before Bottas and Williams teammate Felipe Massa fell outside the top 10 in the afternoon, over half a second slower than both Force India drivers.
"Force India definitively seems very competitive here," Bottas said. "It's going to be a big challenge for us if we want to compete with them -- that's how it feels at the moment, but there's still a long weekend to go."
Williams currently trails Force India by 10 points in the battle over fourth place in the constructors' standings, and while the Grove-based team rarely shows its true pace on Fridays, Bottas conceded that Williams encountered balance issues in both sessions at the Circuit of the Americas.
"In the morning, with the soft compound, it was the best it was all day, by far, but in the afternoon the in-corner balance was not quite together, some entry issues, some snap oversteer in mid-corner and then understeer so we have some work to do.
"In the afternoon, as soon as we went out, we started to struggle with the balance of the car. I don't know, it's difficult to say much because the wind direction is going to change completely for tomorrow so whatever we were doing today, things will be different tomorrow."
A number of drivers struggled to stay on circuit in both sessions in Austin, including the likes of Renault's Jolyon Palmer who spun twice, and Toro Rosso's Carlos Sainz, who ended FP2 going backwards in the gravel in the twisty first sector.
When asked if track conditions were particularly difficult on Friday, Bottas replied: "I think today was tricky with the wind. The first sector was good with the headwind but there was quite a bit of gusty tailwind and it was a bit of a problem.
He added: "But tomorrow it's going to be different, the wind direction will change 180 degrees, so it will make a difference in terms of balance and how the car feels, so whatever we do with the set-up it will be tricky tomorrow."