Valtteri Bottas outpaced Max Verstappen and Lewis Hamilton in a competitive start to the Spanish GP weekend in Practice One at Barcelona.
Bottas, who on Thursday robustly batted away a report suggesting his Mercedes seat was under imminent threat, edged out Red Bull's Verstappen by 0.033s in the opening session, with championship leader Hamilton just over a tenth back in third place.
Mercedes are defending a run of eight successive pole positions at the aerodynamically-demanding circuit and so Red Bull will be buoyed by starting the weekend so close to their chief rivals.
Lando Norris, one of 2021's early form drivers alongside Hamilton and Verstappen, was fourth for McLaren ahead of the two Ferraris of Charles Leclerc and Carlos Sainz.
Aston Martin, meanwhile, finished with both their AMR21s in the top 10, with Sebastian Vettel their lead car in eighth in his first session running the upgrades team-mate Lance Stroll ran first last weekend in Portugal.
Sergio Perez was ninth in the second Red Bull after backing out of his best lap on the soft tyres.
F1 drivers know their way around the Circuit de Catalunya arguably better than anywhere else due to the fact pre-season testing also normally takes place at the circuit, but this weekend marks the sport's first venture to Spain since last August after this year's winter running took place in Bahrain.
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Turn 10 has been reprofiled for 2021, with what was a heavy braking zone replaced by a more flowing left-hander taken at higher speed, with gravel now on the outside of the corner.
And Alfa Romeo reserve driver Robert Kubica, in his first P1 outing of the year, was the first to fall foul of the new corner with the experienced Pole spinning off and beaching his Alfa Romeo inside the session's final 20 minutes.
The incident brought out P1's only red flag.
Nikita Mazepin was the only other driver to spin, although the Haas driver was able to continue his session after dropping it through the Turn Seven-Eight chicane early on.
Second practice starts at 2pm, live on Sky Sports F1.