Mexico City: Lewis Hamilton's hopes of wrapping up his sixth driver's world title this weekend dimmed on Friday when he languished in fifth place behind Sebastian Vettel and Ferrari after second free practice at the Mexican Grand Prix.
After topping the times in the opening session, the defending five-time champion was unable to resist the pace of his main rivals as four-time champion Vettel delivered a best lap in one minute and 16.607 seconds.
"It wasn't good enough," said Mercedes boss Toto Wolff, whose team clinched an unprecedented sixth championship double last time out in Japan.
"This is a difficult track for us, but we are lacking performance. It is one of our worst tracks."
On an overcast but warm day at the Autodromo Hermanos Rodriguez, the outright pace of the Ferraris ruled on the main straight with Max Verstappen, winner of the last two Mexican races for Red Bull, also showing competitive pace.
The Dutchman was second, just one-tenth down on Vettel, but four-tenths ahead of Charles Leclerc with Hamilton's Mercedes team-mate -- and only remaining title rival -- Valtteri Bottas fourth ahead of the champion-elect.
"The Ferraris are too fast for us," said Verstappen. "They will be too quick in qualifying. They are ahead of us all here."
Daniil Kvyat was sixth ahead of his Toro Rosso team-mate Pierre Gasly, Carlos Sainz of McLaren, Nico Hulkenberg of Renault and Lando Norris in the second McLaren.
Wolff admitted that it was unhelpful for Hamilton that he is without his usual race engineer Peter 'Bono' Bonnington in Mexico and Austin next weekend, as he has stayed in Britain to recovery from surgery.
"It's not great, but it is what it is," he said. "Not ideal. It's tough. Rain might be a help here this weekend."
The session was supported by a big crowd and delivered plenty of incident including a 15-minute red-flagged interruption when Red Bull new boy Alex Albon lost control of his car and hit the barriers heavily at Turn Seven, severely damaging the front right section of his car.
"It was a careless moment," admitted the London-born Thai racer.
This crash wrecked his hopes of matching the time of team-mate Verstappen in the rarefied atmosphere of the sprawling Latin American city where, at 2,250-metres, cars and humans struggle to breathe easily.
Tyres were also a problem for most teams as they suffered graining and high levels of wear, conditions that suggest an unpredictable contest lies ahead in Sunday's 71-lap showdown.
Hamilton, who showed strong pace in race trim in the final part of practice, has to outscore Bottas by 14 points to win his third consecutive title in Mexico City and sixth overall.