Ferrari drivers Charles Leclerc and Sebastian Vettel are unsure they will be able to compete with Mercedes' long-run pace in Sunday's United States Grand Prix after a difficult Friday of practice running at the Circuit of The Americas.

Leclerc and Vettel finished P2 and P4 respectively in second practice at COTA on Friday as Lewis Hamilton headed the timesheets for Mercedes, sitting three-tenths of a second clear of the field.

While Leclerc was pleased with how his qualifying pace had stacked up compared to his rivals, he was less confident about his long runs, with averages showing Ferrari to be trailing both Mercedes and Red Bull.

"I think it was a positive day, especially for the qualy pace. I think we were strong today," Leclerc said.

"In FP1 I couldn’t show the real potential, the lap time was deleted and a few things like this, then we had a problem. Overall I think it has been a positive day.

"The race run though is not as positive as the qualy runs, and Lewis seems very strong on that."

Asked if he could take the fight to Mercedes at COTA, Leclerc replied: "In qualifying, I think so.

"In the race, we’ll see. It might be a bit harder."

Vettel echoed his recent comments about Ferrari's one-lap performance masking its deficiencies in race pace, the team having claimed each of the last six pole positions.

"[Friday was] good to start with, that was positive, but then we missed a bit. We tried some things and it didn’t really pay off," Vettel said.

"I think we have a good understanding, hopefully tomorrow we can improve. One-lap we should be there or thereabouts, in race trim we have some work to do.

"In qualy trim we can masks the weaknesses a bit, whereas in the race we struggle, and we can’t. But let’s see.

"I’m quite confident that we can improve the car, that’s what’s most important and what we need to focus on, and then we see what we get."