Mercedes' technical boss James Allison has said the team was forced to protect its engine in Mexico on Friday by turning the performance down, as it struggled with overheating.
Lewis Hamilton and Valtteri Bottas were some way off the pace during first and second practice, with Hamilton finishing seventh quickest and more than 1.3 seconds adrift of the leading Red Bulls. Bottas meanwhile was almost 1.5s off in ninth.
Some of that deficit can be explained by the need to turn their engines down due to insufficient cooling according to Allison, although other factors were also to blame, including degradation on the two softest compounds being far higher than many predicted.
"This is an unusual track, which places unusual demands on the chassis, power unit, cooling systems and tyres," explained Allison, referring to the high-altitude nature of the Autodromo Hermanos Rodriguez.
"From all our running, we have to conclude that we have not yet found the best way to meet those unusual demands with good performance on both single laps and in the long runs.
"We were overheating the power unit in a number of areas, and that meant we had to protect against this by turning it down as a precaution."
Cooler conditions are however forecast for Saturday and Sunday, with the chance of rain increasing over the weekend, meaning engine cooling and tyre usage may not transpire to be such a big issue.
But Allison nonetheless remains wary that Mercedes has to make improvements to get its W09 in the optimum condition.
"With a bit luck, and no little hard work, we can get ourselves into better shape [Saturday] and on Sunday, when conditions are also forecast to be cooler," he said.
"We have plenty of work ahead of us and in FP3 to put ourselves in a stronger position than we were able to achieve [in FP1 and FP2]."
