Lewis Hamilton is wary of a "slap in the face" from the Mercedes W13 on qualifying day after a promising Friday at the Mexico City Grand Prix.
The seven-time F1 champion finished within two-tenths of Carlos Sainz's leading time of a representative FP1 session in which the top six drivers were separated by just 0.192secs.
FP2 gave way to a 90-minute Pirelli 2023 prototype tyre test which, on the face of it, means nothing for this weekend, although team-mate George Russell did at least post the quickest time of the day overall on this year's rubber.
Hamilton set the fastest time of those drivers purely on the testing tyre compounds, beaten only by Russell, AlphaTauri's Yuki Tsunoda and Alpine's Esteban Ocon, all three of whom were provided free running after stepping aside for rookie runs in FP1.
"I feel really good, generally had a really good session," enthused Hamilton.
"I really enjoyed driving around the track. The car has been feeling better and better as we work through it and understand it more.
"Whilst we had the tyre test at the end, it was like reconnaissance laps, just practice, practice, practice.
"I don't know where we stand compared to others because we were on different programmes. George was on a different programme because he missed FP1 but I just focused on mine."
But asked if the form could be carried through the weekend and the performance improved again, Hamilton warned: "I have no idea. We never know with this car.
"Tomorrow morning it could slap me in the face and I will not be happy," he added, chuckling.
"I hope that is not the case, we will do plenty of work tonight to see if we can edge ourselves a little bit forwards."