Daniel Ricciardo set the pace as title rivals Lewis Hamilton and Sebastian Vettel suffered problems in Practice Two for the Mexican GP.
Ricciardo lowered the lap record set by Valtteri Bottas in Practice One to top the timesheet with a 1:17.801.
"I think these last three races will be pretty close between the top three teams," said Ricciardo. "Mercedes always have more on Saturday so I think we still need to find a few tenths if we want to stay ahead of them."
Hamilton was a tenth off the Australian in second - the championship leader recovering from a big spin on his first timed lap of the session - with Max Verstappen a further three hundredths of a second back.
The Dutchman revealed he had tested new-for-2018 components in Practice One and then suffered an engine failure in P2 - although he will avoid a penalty as the unit wasn't his scheduled race engine.
Vettel was fourth but suffered a bizarre problem when the fire extinguisher in his Ferrari car exploded. His Qualifying simulation was also compromised by an error on his first lap where he went wide at Turn 11, so the true pace of his Ferrari has yet to be truly revealed.
Kimi Raikkonen and Valtteri Bottas completed the top six, both less than half a second off Ricciardo to suggest there could be a three-way battle for pole on Saturday.
Fernando Alonso was a very encouraging seventh for McLaren and just seven tenths down on the best time, although the Spaniard will start from the back row after a 20-place grid penalty.
Meanwhile, Romain Grosjean of Haas and Toro Rosso newcomer Pierre Gasly will enter Saturday with almost no running under their belts having sat out Practice One and suffered issues early in the afternoon session.
Grosjean span on his second lap and the resulting puncture caused terminal damage to his Haas and brought out an early red flag.
Gasly, meanwhile, was limited to just 10 laps by a power unit issue. Verstappen's session was also brought to a premature close by an engine problem, although Red Bull team principal Christian Horner confirmed to Sky F1 it was not his race power unit.
Hamilton's strategy compromised?
Hamilton is guaranteed to win his fourth world championship on Sunday provided he finishes in the top five, while Vettel must outscore him by 17 points to take the title race to Brazil.
But the Briton's race strategy may be compromised after his uncharacteristic mistake on his first timed lap on the supersoft tyre.
Hamilton lost his car through the exit of Turn 11 and was fortunate to keep it out of the wall but the lack of running on the supersoft will have left Mercedes with a lack of data.
"This doesn't give the Mercedes the ideal scenario," said Sky F1's Paul Di Resta.
"They can't put all the information into the computers and dictate what the best strategy is. They don't know what that supersoft is going to do."
Vettel, meanwhile, made a belated start to the session as Ferrari finished making set-up changes to his car.
The German was then forced into an early return to the pits and a change of overalls after his extinguisher leaked under his seat.
And on his opening flying lap on the ultrasofts, Vettel went off at the same place as Hamilton before ending up 0.250 seconds off Ricciardo.