Max Verstappen underlined the threat Red Bull seem poised to carry to Mercedes when the F1 season begins in two weeks' time by topping the final day of testing in Bahrain.
On the day Lewis Hamilton predicted Red Bull would be a "different animal" this year after an impressive pre-season, Verstappen set the quickest time of the shortened three-day programme with a lap of 1:28.960.
Team-mate Sergio Perez had topped the morning session in the height of the day-time conditions at Sakhir.
Hamilton and Mercedes were fifth, one second off the pace, and the seven-time champion also spun late on.
All of the leading times came in the closing hour of the session under Bahrain's floodlights, with teams trying shorter simulations with the quickest tyres on what would have been varying levels of fuel.
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"This test has shown us that the order has been mixed up a little bit, and to an extent, closed up," said Sky F1's Martin Brundle ahead of the season-opening Bahrain GP on March 26-28, live only on Sky Sports F1.
Japanese rookie Yuki Tsunoda, who has impressed in his first pre-season, ran Verstappen closest on the timesheet with a series of quick laps in an AlphaTauri car that has also caught the eye this week.
Ferrari's Carlos Sainz and Alfa Romeo's Kimi Raikkonen were third and fourth respectively but, in a rarity for testing, banged wheels amid an apparent track disagreement in the closing seconds of running.
Has the gap closed to Mercedes?
Although Mercedes ostensibly finished the test one second behind the pace of Red Bull, the reality is that F1's dominant force of the last seven years will be rather closer than that when the racing season begins back at the same venue.
However, the first test with their new W12 has undoubtedly proved trickier than the early running with most of its title-winning predecessors in the hybrid-turbo era.
Mercedes completed a healthy haul of 140 laps on Day Three between Hamilton and Valtteri Bottas, who ended up 16th after running in the hotter morning session, but their Day One gearbox delay meant they still ended up bottom of the combined mileage table for the three days.
Hamilton also spun for the second successive day, this time at the final corner, although he was able to continue without further delay.
But it still appeared that Mercedes had made improvements to the W12's handling since the opening day, with Sky F1's Karun Chandhok suggesting the car was looking more together when he was watching close-up trackside.
Problems for Ferrari and Aston Martin
Ferrari and Aston Martin both ran into reliability problems in the final afternoon, a particular blow to the latter who had also experienced delays with the AMR21 on the opening two days.
New signing Sebastian Vettel started the afternoon strongly, completing 56 laps on high fuel, but lost much of the final hour when the track was at its best due to a loss of boost pressure on his Mercedes-powered car.
The four-time champion completed only 117 laps for his new team across the three day test, the fewest of the grid's 20 race drivers. AlphaTauri's Pierre Gasly topped the charts with 237 laps.
Sainz lost track time due to a hydraulics issue on his SF21, although the new Ferrari arrival did return to the track in the closing stages to post the third-quickest lap on his qualifying simulation.
McLaren, meanwhile, concluded a successful test for their new Mercedes-powered car - which has caught the attention of the paddock due to its innovative diffuser decision. Daniel Ricciardo was seventh, with Lando Norris 11th, as they combined to complete more than 130 laps.
Alpine again completed a lot of laps - 61 for Esteban Ocon in the morning and 78 for Fernando Alonso in the afternoon - but appeared adrift of some of their midfield rivals on pace.