George Russell topped the timesheets during the first free practice for the Sakhir Grand Prix.
The 22-year-old, seated in Hamilton's Mercedes after the Briton tested positive for Covid-19, lapped the 3.5-km outer circuit of the desert track that also hosted last weekend's Bahrain Grand Prix in 54.546 seconds.
Red Bull's Max Verstappen and Alexander Albon were second and third respectively, with Mercedes' Valtteri Bottas only managing fourth, 0.322 seconds off the pace.
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However, the Finn was nursing damage on his car.
"Valtteri broke his car pretty early on in the session," Mercedes team principal Toto Wolff said.
"He even stopped running.
"So we didn’t see Valtteri in the right car, but this is not to diminish George’s performance for a first time in the car, it’s good.”
Behind the top four, Russian Daniil Kvyat was fifth ahead of AlphaTauri team mate Pierre Gasly.
Esteban Ocon was seventh for Renault ahead of Ferrari's Sebastian Vettel and team mate Daniel Ricciardo.
Charles Leclerc in the other Ferrari rounded out the top 10.
Russell, whose is backed by Mercedes but had been placed at engine customer Williams to learn the ropes for the last two seasons, was not the only driver getting used to new surroundings.
Brazilian Pietro Fittipaldi is making his Formula One debut at Haas as stand-in for the injured Romain Grosjean.
Briton Jack Aitken, also a rookie, is filling Russell's seat at Williams.
The pair propped up the bottom of the timesheets.
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