Lewis Hamilton finished fourth in practice ahead of his bid to win a record-equalling world championship at Sunday's Turkish Grand Prix.
Hamilton can emulate Michael Schumacher by securing his seventh title if he takes the chequered flag for a 10th time this season in Istanbul on Sunday.
But the Mercedes star, only 15th in the opening running here, ended the first day on Formula One's return to Turkey 0.850 seconds behind pacesetter Max Verstappen.
Red Bull's Verstappen completed a practice double on Friday to suggest he could deny Hamilton the chance to take the championship in style.
Ferrari's Charles Leclerc was second ahead of Valtteri Bottas. The Finnish driver, 85 points adrift in the standings, is the only man who can stop Hamilton from winning the title. Hamilton will be crowned champion if he finishes Sunday's race ahead of Bottas.
Mercedes have dominated the season with the Silver Arrows starting every race from pole and taking 11 wins from 13 rounds.
But the sport's all-conquering team and their star driver might not have it all their own way here.
The sport last staged a race in Turkey back in 2011 and the fast-flowing Istanbul Park circuit resembled an ice rink as the drivers failed to get sufficient temperature into their tyres on the resurfaced tarmac.
Although the conditions improved as the asphalt was rubbered in, Hamilton spent much of the second session bemoaning a lack of grip. He finished almost three tenths behind Bottas in the other Mercedes.
Verstappen has consistently out-performed his Red Bull machinery this year, and the Dutchman will hope he can build on his practice speed to take a second win of the campaign.
There will be some joy for Ferrari, too, with Leclerc four tenths back in second.
The Scuderia are sixth in the constructors' championship following a miserable year. Leclerc's team-mate Sebastian Vettel finished eighth, more than one second off Leclerc's pace.
Lando Norris, who turned 21 on Friday, ended the day in 10th for McLaren.
His compatriot George Russell, who is facing a grid penalty after exceeding the permitted number of engine parts allowed, finished 17th in his Williams.