Despite his subsequent disqualification for a technical breach, Lewis Hamilton had been delighted with his second place finish in the US and hoped for more this weekend in Mexico - only to find the W14 behaving completely differently.
"It's been a challenging day for me today," admitted the Mercedes driver, who was outside the top ten in first practice for the Mexican Grand Prix, but rallied to finish in P7 in the later one-hour second session. “Not that great. Dodgy - not dodgy, but not the greatest. A bit of a struggle in the car today.
"The car felt completely different to what it felt like in Austin a week ago and we need to understand why," he continued. "The car is night and day different compared to last week. I don’t know what 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. There is definitely performance there, it’s just hard to extract it and it’s quite peaky this weekend with the aero map, or whatever it might be.
"We didn't show the most competitive long-runs so we've been focusing on trying out different set-ups," he explained. "We're not yet fully where we want to be ahead of tomorrow, but we'll work hard overnight to improve and make the necessary changes.
"The track here comes with its challenges, but we've done well here in previous years," he added. "The car showed a promising pace in Austin and I'm confident we can get it to a good place here and there's plenty of room for us to improve."
Asked what he expected to achieve in qualifying, Hamilton replied: “I’m not sure. Again, we never know what to expect with this car. Maybe we make the changes and we’ll pick that pace up.
"There was definitely some performance in there to be closer, maybe in the top three. But we’ve got to figure out how to make it easier for us to extract that performance, so that’s what we’ll be working on tonight.”
His team mate George Russell had challenges of his own, stepping out of the car in FP1 to allow young driver Frederik Vesti to take the wheel for the session, before getting back up to speed himself in the second session.
"It felt good to be back in the car, but I had a challenging session today with mixed weather conditions and a bit of rain towards the end of FP2, which compromised the consecutive laps for everybody."
His lack of running time today contributed to finishing the day in tenth place, and Russell admitted that there were a number of drivers ahead of him that he hadn't expected to be quicker such as Daniel Ricciardo and Valtteri Bottas.
“It’s very tight, especially with some unexpected cars, to be honest, some cars in the top ten that we weren’t expecting to be there. We feel like we’ve got a lot to improve, but we’ve got the opportunity to do it as well."
He added that there had been "lots of traffic during the sessions today," noting that this was "one of the challenges we will need to tackle this weekend."
"We also need to make some improvements with the pace of our car if we want to be at the front end of Q3 tomorrow," he said. "We’ve been probably in the wrong window today, I think. Lewis and I were running two different setups and we probably both weren’t in the right window.
“That’s given us an indication that probably halfway between both is the right place to be. So that's frustrating in the moment to have a negative car and not have the best of days, but through those difficult moments you learn probably more than when the car is in a better place.
"That’s what practice is all about," he said. "I know we will work hard overnight to achieve that."
"We've struggled with the car today," agreed Andrew Shovlin, trackside engineering director for Mercedes after the end of Friday practice at the Autódromo Hermanos Rodríguez.
"Lewis was finding the car quite a handful, he admitted. "We've seen enough to know that we have work to do on both single lap and long run. George is still getting up to speed having missed the first session, and both lost a bit of time on single lap with traffic.
"It's also interesting that most teams have had a driver featuring near the front at some point today, which almost certainly means that getting through to the final part of qualifying will not be comfortable.
"The balance has been pretty poor throughout the day, which is normally a good indication there is lap time that can be found," he added. "We've got some good lines of enquiry to follow up overnight so we'll keep working on it."