Ferrari's Sebastian Vettel was a surprise pace-setter in second practice at the Mexican Grand Prix.
The four-time champion upstaged title rivals Nico Rosberg and Lewis Hamilton, pipping the Briton by 0.004 seconds.
Rosberg, who can tie up the title on Sunday if results go his way, was 0.435secs behind Vettel.
The German, 26 points ahead with 75 available over the remaining three races, will be champion if he wins the race with Hamilton 10th or lower.
Rosberg can afford to finish second - or second twice and third once - even if Hamilton wins all the remaining races and still be champion.
Ferrari's Kimi Raikkonen was fourth fastest, with Red Bull's Daniel Ricciardo, poised to secure third place in the championship behind the Mercedes drivers this weekend, only fifth and team-mate Max Verstappen seventh behind Force India's Nico Hulkenberg.
Final practice (16:00 BST) and qualifying (19:00) are live on the BBC Sport website and radio 5 live sports extra.
Could Ferrari put a spanner in the works?
If Ferrari are in the mix at the front on Sunday, it would be unexpected after a difficult season for the Italian team, but it could play into Hamilton's hands.
Because Rosberg has such an advantage, Hamilton ideally needs him to finish below the podium positions. If Hamilton won with Rosberg lower than third, the Briton would be able to control the championship - two wins in the final two races would be enough, even without any further problems for the other Mercedes.
Ferrari have been working hard on developing their car in recent races and the SF16-H tends to go better relative to Mercedes at the races where the softer tyres of the five in the Pirelli range are being used.
Mexico is one of those tracks, with the super-soft, soft and medium being the tyres available.
It was harder than usual to get a read on relative form because Ferrari did not run the super-soft tyres at the start of the race-simulation runs.
Instead, they sent Vettel out on the medium tyre and Raikkonen on the soft, while Mercedes and Red Bull did the more usual thing in running the super-soft.
Of the super-soft runners, Hamilton was comfortably quicker than Rosberg, who was closely matched with the Red Bulls.
Hamilton, who was beaten to pole and win by Rosberg in Mexico last year, said he had had "a solid day", adding: "The track is feeling better than it did last year and I feel a lot more comfortable in the car than I did the first time we came to this circuit, so hopefully I can carry that through the rest of the weekend.
"I didn't really get the maximum out of my lap and out of my tyres so I think we have got more pace."
Asked if Ferrari were a big threat, Hamilton said: "I don't think so but we'll find out tomorrow."
"It was a good day," said Vettel. "The car seems to work, but there was a lot of traffic - I felt like I was in Mexico City, not on the race track.
"Lewis' lap didn't look particularly good so I think he has a little more in the pocket. We need to be realistic. If it's like this tomorrow, we take it for sure."
What about Red Bull?
Red Bull have been Mercedes' closest challenger in recent races but they appeared off the pace on one lap on Friday. However, Verstappen said he had struggled initially to find a good set-up after missing first practice following a brake fire. He said the short runs were "not representative" as a result but that "our race pace was very strong".
He added: "Ferrari look definitely very strong but we just need to work hard, understand the tyres a bit more on this slippery track and then I am pretty sure we are there as well tomorrow."
A topsy-turvy timesheet
The whole day was a difficult one to read, perhaps as a result of the low-grip track surface at an unexpectedly cool Autodromo Hermanos Rodriguez, where highs were only 16C.
Tyre supplier Pirelli said it was difficult to draw conclusions from the day's running and that the competitive picture would be clearer once more rubber had been laid down on the track on Saturday morning.
Nico Hulkenberg's Force India split the Red Bulls, Williams' Valtteri Bottas was eighth quickest and Fernando Alonso's McLaren was 10th fastest on a track where the Honda-powered team were expecting a difficult weekend.
Regardless, it did appear as if Hamilton had an advantage in pace over Rosberg.
He was 0.7secs quicker in the first session in addition to his smaller advantage in the second and the auguries appear good at this early stage for him to keep the title fight alive, barring some kind of technical problem or error.