It was a low-profile opening day in Mexico City for Lewis Hamilton and Valtteri Bottas, with Mercedes forced to turn down the performance of its engine due to overheating concerns.
The Silver Arrows' drivers concluded FP2 in 7th and 9th place, well off the pace of the leading Red Bull duo of Max Verstappen and Daniel Ricciardo.
According to Mercedes technical director James Allison, the Mexican heat coupled with the rarefied high altitude air conditions are challenging the cooling of the W09's power unit.
"This is an unusual track, which places unusual demands on the chassis, power unit, cooling systems and tyres," explained Allison.
"From all our running today, 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 today, and that meant we had to protect against this by turning it down as a precaution.
"With a bit luck, and no little hard work, we can get ourselves into better shape tomorrow and on Sunday, when conditions are also forecast to be cooler," he added.
"We have plenty of work ahead of us overnight and in FP3 tomorrow to put ourselves in a stronger position than we were able to achieve today."
Hamilton's concerns were mainly centered around tyre degradation.
"We run the biggest wings at this track but are still quite quick on the straights because of the altitude, but then there's not a lot of grip through the corners, even on the hypersoft tyres," said the Brit who is hoping to secure his fifth world title on Sunday.
"We've got some ground to catch up on, so we're now going to go through everything in the debrief and look at all the details to find some answers."
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