Valtteri Bottas, Lewis Hamilton and Max Verstappen carried on the trend of 2020 in leading the field in first practice at the Portuguese Grand Prix.
On F1's first visit to the undulating Autodromo do Algarve, Bottas was 0.339 seconds ahead of Hamilton.
Verstappen was 0.781secs off the pace, and narrowly ahead of Ferrari's Charles Leclerc in fourth.
Both Verstappen and Leclerc had spins early in the session as they learned the demanding new track at Portimao.
Red Bull's Alexander Albon was fifth fastest, ahead of McLaren's Carlos Sainz, Racing Point's Sergio Perez and Alfa Romeo's Kimi Raikkonen.
Renault's Daniel Ricciardo and Alpha Tauri's Pierre Gasly completed the top 10, ahead of Ferrari's Sebastian Vettel and McLaren's Lando Norris.
There were spins as well for Raikkonen, Haas driver Romain Grosjean, Alpha Tauri's Daniil Kvyat and Williams' Nicholas Latifi.
There was a dash of light rain during the session, but nothing to disturb the drivers on a track many approached with high anticipation.
Portimao, which is hosting Portugal's first F1 race since Estoril in 1996, has been likened to a rollercoaster for its dramatic changes of elevation, which are particularly notable in the the second half of the lap.
Although Mercedes were comfortably clear of the field, Bottas and Hamilton set their fastest times on the medium tyre, while Verstappen, Leclerc and Albon were on the hard.
Red Bull have been slowly closing in on Mercedes over the second half of the season, following the world champions' decision to end development on this year's car and focus on their 2021 design as a result of the huge performance advantage they showed through the first half of the season.
Ferrari have the latest in a series of aerodynamic upgrades this weekend, adding a redesigned floor to the frontal aerodynamic changes around the front wing and nose that they have introduced over the last couple of grands prix.
Renault's Esteban Ocon suffered an engine problem right at the end of the session.