Fernando Alonso edged out Max Verstappen in final practice for the Bahrain Grand Prix to set up a thrilling and unpredictable first battle for pole position of the season, which may also involve Mercedes.
Alonso, backing up his stunning form from Friday evening, topped Verstappen's best time on the soft tyres by five thousandths of a second towards the end of the hour-long session in the baking Sakhir heat.
Sergio Perez was a tenth back in the other Red Bull, just ahead of a much-improved Mercedes.
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Lewis Hamilton had earlier led practice and finished fourth, just two-tenths off Alonso's marker.
Charles Leclerc was fifth in the lead Ferrari ahead of George Russell in the other Mercedes, hinting at an epic four-team battle for the front positions in qualifying, which starts at 3pm live on Sky Sports F1.
"This is just absolutely thrilling," said Sky Sports F1's Nico Rosberg.
"Fernando Alonso, 41 years old in an Aston Martin that was just nowhere last year... and suddenly they're fighting right up there.
"Everyone was going for it in that session. This is going to be so interesting in qualifying.
"Max did make a bit of a mistake at the last corner. But that gap, is just nothing."
Ferrari's Carlos Sainz and Aston Martin's Lance Stroll were next, while F1's new 'best of the rest' position - ninth - was filled by the McLaren of rookie driver Oscar Piastri, four places ahead of team-mate Lando Norris.
The Alpines sandwiched the McLarens, along with Alfa Romeo's Zhou Guanyu, while the Haas cars fell well out of the top-10 after shooting up into fifth in Friday's second practice.
Nyck de Vries was last for AlphaTauri before the opening qualifying session of F1 2023, which will set the grid for Sunday's race.
Formula 1 is back! Watch the season-opening Bahrain Grand Prix live on Sky Sports F1 this weekend. Sunday's race begins at 3pm, with build-up from 1.30pm. Get Sky Sports