Ferrari's Sebastian Vettel set the pace in practice for the Mexican Grand Prix as Formula One championship leader Lewis Hamilton was only fifth fastest.
Hamilton can claim a sixth title and a third on the bounce on Sunday if he beats Mercedes team-mate Valtteri Bottas by at least 14 points.
However, neither driver was particularly impressive on what was a frustrating day for the Mercedes team at the Autodromo Hermanos Rodriguez on Friday.
Former champion Vettel clocked in at 0.115 seconds faster than Red Bull's Max Verstappen, with Ferrari's Charles Leclerc in third position on the time sheets.
Vettel was 0.614 seconds better off than fourth-placed Bottas, with Hamilton 0.963secs adrift of first place.
Mercedes did look stronger on race pace, however, with Hamilton, who was operating without his usual race engineer Peter Bonnington, coming in quicker than Vettel on average.
Though Verstappen produced a strong performance, his Red Bull team-mate Alexander Albon endured a difficult session, crashing out early on.