Max Verstappen made it a hat-trick of fastest times by setting the pace in a damp final practice session for the Mexican GP, but the chasing pack closed on Red Bull significantly to hint at an intense qualifying battle.
While Red Bull had it all their own way on Friday, Verstappen only outpaced Lewis Hamilton and Sebastian Vettel by two tenths of a second in Practice Three.
That improvement may have something to do with the change in conditions - heavy morning showers in Mexico City meant there was a frantic end to P3 as drivers waited for the track to dry out - but Mercedes and Ferrari's times were still more than a second quicker than they had managed on day one.
"The conditions have definitely brought Mercedes and Ferrari back in it," said Sky F1's Paul Di Resta.
Practice Three results in full
Mercedes, however, have major reliability fears ahead of qualifying, which starts at 7pm on Sky Sports F1, after Valtteri Bottas pulled up with a hydraulic failure towards the end of the session.
"We are not in a happy place," Toto Wolff told Sky F1, with the Mercedes boss also claiming the Finn is set for an "engine swap".
Verstappen led the way with the fastest time of the weekend, a 1:16.284 on hypersoft tyres, impressively outpacing team-mate Daniel Ricciardo by seven tenths of a second.
The Australian finished fourth, just ahead of Kimi Raikkonen and Sauber surprise package Charles Leclerc.
Leclerc's form will be a worry for many of the midfield teams, including Renault who were Red Bull's closest challengers on Friday.
Carlos Sainz and Nico Hulkenberg were seventh and 10th respectively in final practice.
"This is going to be a thrilling qualifying - and just what we wanted," added Sky F1's Simon Lazenby.
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