Max Verstappen was fastest in the first practice session at Bahrain ahead of the Formula One season start.
The Dutch youngster , who was fastest overall in pre-season testing at the same Sakhir circuit, set a best time of one minute 31.394 seconds.
Valtteri Bottas, who has won the opening race of the championship for the past two years, was 0.298 seconds slower for champions Mercedes.
Saudi Arabian Grand Prix
'Just want a clean championship' claims Wolff; Horner says he didn't 'know what Hamilton was doing'
McLaren's Lando Norris was third fastest, with a car now powered by a Mercedes engine after the switch from Renault, with seven times world champion and fellow Briton Lewis Hamilton fourth for Mercedes, 0.527 off Verstappen.
Norris had gone top with 27 minutes to go before Bottas went faster and then Verstappen, who reeled off a string of fastest sectors in the closing minutes.
Friday's two practice sessions are now each one hour long, rather than the previous 90 minutes, in a change for 2021.
Ferrari's Charles Leclerc was fifth on the timesheets with Mexican Sergio Perez, Verstappen's new team mate, sixth and Pierre Gasly seventh for AlphaTauri.
Carlos Sainz, switching from McLaren to Ferrari, was eighth with Australian Daniel Ricciardo, taking Sainz's place at McLaren, ninth.

'I don't quit' - Hamilton on Mercedes deal and F1 challenges

Saudi Arabian Grand Prix
‘That guy is f*****g crazy’ – Hamilton rages after Verstappen incident
Saudi Arabian Grand Prix
Hamilton wins drama-filled Saudi Arabian GP to set up thrilling season finale