Red Bull's Max Verstappen was fastest ahead of Lewis Hamilton as Mercedes had a messy final practice session at the Mexican Grand Prix.
Hamilton and team-mate Nico Rosberg were both held up by traffic and not able to get a clean run on super-soft tyres at the end of the session.
Hamilton was 0.094 seconds slower than Verstappen, with Rosberg 0.481secs off.
Rosberg will be world champion if he wins Sunday's race and Hamilton finishes 10th or lower.
The German is 26 points ahead of Britain's Hamilton with 75 available over the remaining races. Hamilton needs to win this race to keep a realistic hope of staying in the battle - and even then Rosberg will be champion if he finishes at least second twice and third once in the remaining races.
Qualifying gets under way at 19:00 BST, with build-up on the sport website from 18:00 and on BBC Radio 5 live sports extra from 18:55.
But the events of Saturday morning in Mexico City, where the weather was sunny but a chilly 15C, suggest that Mercedes might at least face a battle this weekend.
Rosberg has appeared off Hamilton's pace all weekend - the near half-second margin is what the gap has been in all three practice sessions so far.
If that margin remains, it could be a lifeline for Hamilton's title hopes. Because of Rosberg's mathematical advantage in the championship, the Briton needs other teams to get between him and his team-mate if he can win the race.
Red Bull's Daniel Ricciardo was third, 0.233secs behind Verstappen after experiencing traffic on his fast lap, while behind Rosberg, Williams' Valtteri Bottas was fifth ahead of the Ferraris of Sebastian Vettel and Kimi Raikkonen.
Vettel had been fastest of all on Friday afternoon in second fastest but that pace seemed to have evaporated in the different conditions of Saturday morning.
Jenson Button was 15th fastest, two places and 0.552secs behind McLaren team-mate Fernando Alonso and complaining of traffic.
Jolyon Palmer was 14th, four places and 0.386secs ahead of team-mate Kevin Magnussen as both fight for their futures in F1 next year.
Magnussen, as has become something of a habit this season, incurred the wrath of several other drivers for apparently not paying attention to his mirrors out on track.