Just five days after finishing a close second on the road to Max Verstappen at COTA, Hamilton was 11th in FP1 and seventh in FP2, 0.338s off the pacesetting Red Bull driver in a tightly-packed field.
His team-mate George Russell was 10th in FP2, having handed his car to Mercedes junior driver Frederik Vesti for the first session.
After some changes in the break, Hamilton and Russell ran different set-ups in the later session, and neither was happy with the performance.
"Not that great,” said Hamilton when asked about his Friday. “Yeah, dodgy - not dodgy, but like just not the greatest. Bit of a struggle in the car today.
“The car is night and day different compared to last week. I don't really know what really to say.
"You just never know what you're going to get with this one. Some days, she’s great, and some days, she's not. It's hard to extract the lap.”
Hamilton remains confident that the car can be improved for Saturday.
“I think there is definitely performance there," he said. "It's just trying to extract it, and it's quite peaky this weekend, maybe just with the aero map or whatever it may be. So we're going to work on it overnight.
“But definitely it wasn't a fun day compared to FP1 in Austin. And, yeah, we're a bit a bit off, but hopefully overnight, we can find something and tomorrow [Saturday], maybe the car will be nicer to drive."
Asked how much could be extracted from the car, he said: "Not sure. Again, you just never know what to expect with this car. And maybe we'll make the changes, and we'll pick that pace up.
“I think there was definitely some performance in there to be closer, and maybe in the top three.
"But we've got to figure out how to make it easier for us to be able to extract that performance. So that's what we'll work on tonight."
Russell, who had to play catch up after missing FP1, noted that there were a lot of factors at play in the disappointing performance. Like Hamilton, he believes that the car can be improved overnight.
"It was a bit of a tricky session, there was rain around here and there, we had the prototype Pirelli tyre, and it's a very tight field out there,” he said.
“But I think there is potential, I think we've got a lot of work to do overnight, it's very tight.
"Especially with some unexpected cars, you saw some cars in the top 10 that we weren't expecting to be there, but we feel like we've got a lot to improve. But we also feel like we've got the opportunity to do it as well."
Russell believes that the difficult Friday has pointed the team in the right direction for Saturday.
“I think we've been probably in the wrong window today,” he said. “Lewis and I were running two different setups, and we probably both weren't in the right window, so that's given us an indication that probably halfway between both is the right place to be.
“It's frustrating in the moment to have a sort of negative car, and not have had the best of days. But through those difficult moments you learn probably more than when the car is in a better place. So I guess that's what practice is all about.”
He added: “Of course, we want to be fighting for pole position, Max [Verstappen] always is half a step ahead, but I think I think anybody if you get it right can be fighting for the front two rows. We need to make sure that's us."
