Mexico City: Red Bull's Daniel Ricciardo reminded Lewis Hamilton on Friday that he has a fight on his hands to win the Mexican Grand Prix however inevitable the Mercedes's driver's fourth world championship may be.
Hamilton is on the brink of becoming Britain's first four times world champion but he wants to do it with a victory on Sunday.
He is 66 points clear of Ferrari's Sebastian Vettel and needs only to finish fifth on Sunday to secure the title with two races to spare, even if the German wins.
The 32-year-old was the meat in a Red Bull sandwich in second practice at the Autodromo Hermanos Rodriguez, however, with Ricciardo top of the timesheets with a lap of one minute and 17.801 seconds.
Hamilton had a big spin and was 0.131 slower with Red Bull's Max Verstappen third, 0.163 off his team mate's time, despite an engine failure with an older power unit that was of no concern.
Vettel was fourth, and also with a hot backside after his car's fire extinguisher went off beneath his seat half an hour into the session.
"On the long runs, I still think we could find a bit more," said Ricciardo.
"On low fuel we are more or less there. 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.
"I feel we can be a bit better on race pace and I think we can improve a few things. I think these last three races will be pretty close between the top three teams."
Finland's Valtteri Bottas had been fastest in the sunny morning session with a best lap of 1:17.824 also keeping Hamilton off the top. Verstappen and Ricciardo were third and fourth respectively.
Hamilton's 2016 pole position time was 1:18.704.
The Briton said it had been a good day, even if it did not start out that way and he had put himself on the back foot by damaging a set of supersoft tyres in the spin.
"But the single lap and then the long run on the ultraSoft was probably one of the best I've ever done. I don't think I've ever done 26 laps that consistent, apart from in the race maybe," Hamilton said.
"We got lots of information, the track was feeling better in the second session once the temperature came up," added last year's race winner.
Sergio Perez was seventh fastest for Force India before lunch, with regular team mate Esteban Ocon sitting out the session to make way for Mexican Adolfo Celis Jr. to give the home crowd two drivers on track.
Ocon will not have enjoyed being a spectator as Celis put the Frenchman's car backwards into the barriers at turn 16 and halted the session. Ocon returned to the cockpit after lunch and was 10th fastest.
New Zealander Brendon Hartley, preparing for his second race with Toro Rosso, managed only 10 laps in the morning with mechanics working on the car after he stopped on track. He was 13th fastest in the afternoon.
There were also more problems for McLaren, with Belgian Stoffel Vandoorne sidelined completely as the team changed his car's power unit to a spare. He already has a 35-place grid penalty.