Sebastian Vettel edged out Red Bull's Max Verstappen to underline Ferrari's status as favourites for the Mexican GP - as world champions Mercedes struggled behind their big two rivals.
Having predicted that the unique high-altitude challenge of Mexico City might again cause them problems, Mercedes finished an unusually large margin off the leading pace on Friday as Ferrari and Red Bull led the way.
"Not good enough. We know this is a difficult track for us. We are just lacking the performance," Mercedes boss Toto Wolff told Sky Sports F1.
Vettel's pacesetting lap of 1:16.602 was only 0.115s faster than Verstappen but the lead Mercedes of Valtteri Bottas was 0.614s adrift.
Lewis Hamilton finished nearly one second off Vettel's pace in fifth. The Briton can clinch a sixth world title in Sunday's race, but what hitherto appeared an uphill prospect of outscoring Bottas by 14 points - which would require a podium finish at the very least - appears even more challenging now on this evidence.
"Mercedes way down compared to what you would expect," Sky F1's Paul Di Resta said.
But it was not all plain sailing for Mercedes' rivals either in Mexico's second session.
Charles Leclerc spun twice in the second Ferrari and finished nearly half a second slower than the in-form Vettel in third place behind Verstappen.
But Alex Albon endured a more problematic session in the other Red Bull, spinning into the barriers after just five laps and causing session-ending damage to his car.
Toro Rosso, meanwhile, enjoyed an impressive start to the weekend with Daniil Kvyat and Pierre Gasly finishing ahead of the McLarens and Renaults in sixth and seventh places respectively.
Can Ferrari be caught?
Unbeaten in qualifying since the August summer break, Ferrari have now consistently headed the field on outright single-lap pace for two months - even if Mercedes have won the last two races.
The gap between the Scuderia and Verstappen's Red Bull was relatively small at the end of P2, but the latter were firmly of the opinion that Ferrari remain the team to beat.
"I don't think we can fight with them in qualifying," Verstappen said. "They are just too quick.
"The rest is pretty much in the same speed range but they are just miles ahead [over one lap]. That's the way it is at the moment and we just can't gain the time back in the corners unfortunately."
But the race, as in Sochi and Suzuka, could again represent a different proposition - particularly with most cars suffering from tyre graining in Friday's long runs.
"Tyres are completely gone," Leclerc reported as he neared the end of a stint on the soft tyres.
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