Lewis Hamilton bemoaned the difference in how his Mercedes performed after practice ahead of the Mexico City Grand Prix compared to last weekend in the United States.
In the Sprint weekend in Austin, Hamilton qualified for both races in third and finished both, on the road at least, in second before his Grand Prix disqualification.
Hopes were high of being able to take the fight to the all-conquering Max Verstappen and Red Bull at the high-altitude Autodromo Hermanos Rodriguez - but Hamilton felt the W14 was "night and day different" in its performance in Mexico.
He finished the second practice session in seventh, although just 0.338s down on Verstappen's benchmark time.
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Hamilton confused by Mercedes in Mexico
"It was not that great, it was a bit of a struggle in the car," the two-time Mexico winner explained.
"The car is completely night and day different compared to last week, and I just don't know what to say. You never know what you're going to get with the car.
"Some days she is great, and some days she is not. It is hard to extract the lap, but I think there is definitely performance there - so it is just trying to extract it.
"It is quite tricky this weekend, maybe with the aero map or whatever it may be, so we're going to work overnight but it wasn't a fun day.
"Compared to Austin, we're a bit off but hopefully we can do something and the car will be nicer to drive.
"I'm not sure what to expect [heading into qualifying] - maybe we will make the changes and pick that pace up - there was definitely some performance in there to maybe be closer to the top three - but we've got to figure out how to make it easier for us to be able to extract that performance."