Britain's George Russell set the fastest time in first practice at the Sakhir Grand Prix as he had his first miles in Lewis Hamilton's Mercedes.
The 22-year-old, deputising after Hamilton contracted coronavirus, was 0.176 seconds quicker than Red Bull's Max Verstappen.
The second Mercedes of Valtteri Bottas was fourth, 0.322secs off the pace.
The Finn made a series of mistakes on his laps and damaged his car in an off-track moment at Turn Eight.
Hamilton recovering but 'not feeling great'
Sakhir Grand Prix - first practice results
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Russell, who is a Mercedes protege, has been driving for Williams since he made his F1 debut in 2019.
Mercedes team boss Toto Wolff decided he wanted to try Russell out in the car in Hamilton's absence rather than designated reserve driver Stoffel Vandoorne.
"P1 was a good session for him," said Wolff. "We need to calm everybody down - it was a first practice session on a really short track. But he delivered a really solid job in what we expect from him on a single lap.
"The long runs were difficult with our car and difficult to establish a benchmark because [Bottas] broke his car early on and it was difficult for him to stop it properly."
Russell, who has been a Mercedes young driver since 2017, has done a total of just over 6,500km of testing for the team but had not driven this year's car for real or in the simulator before this practice session.
He revealed on Thursday that he is having to wear driving boots one size too small to fit his size 11 feet in the car.
At 6ft 1in tall, Russell is five inches (14cm) taller than Hamilton.
He spent his first run acclimatising to the car and ended it a second slower than Bottas.
But on their second runs Russell went to the top of the timesheets on his first flying lap and Bottas started to make mistakes, apparently over-driving as he sought to match Russell's time, running wide at Turn Eight and having a wobble through Turn Two.
On his second flying lap of the run, Russell almost matched his first time, and then on his third he knocked a further 0.3secs off to end the session in impressive style.
His friend Charles Leclerc, the Ferrari driver, predicted on Thursday that he would "bet on a win" for Russell, even if it was "very optimistic", but the prospect now looks far more realistic than it did before cars started to run on track.
BBC F1 analyst Jolyon Palmer said: "He's built up to it far quicker than I expected him to, but let's just hold judgement a little bit longer."
The race is taking place on a previously unused, shorter version of the Sakhir track, which contains only four real corners.
Russell's fastest time was 54.546 seconds - the shortest lap of the season by far - which is likely to lead to a hectic weekend.
Behind Bottas, Alpha Tauri's Daniil Kvyat and Pierre Gasly were next, ahead of the Renault of Esteban Ocon, Ferrari's Sebastian Vettel, the second Renault of Daniel Ricciardo and the second Ferrari of Charles Leclerc.