Nico Rosberg gave a hint of Mercedes' true speed on Tuesday with the fastest time on the first day of Formula One's final pre-season test.
It was the first time in 2016 testing that the champions had led the timesheets at the end of the day and Rosberg was not even using the quickest tyres to go faster than everyone else.
The German lapped the Circuit de Catalunya with a best time of one minute 23.022 seconds after using soft tyres for the first time this year.
He was two-tenths of a second faster than Valtteri Bottas (1:23.229) who was using the ultra-soft tyres on his Mercedes-powered Williams.
McLaren's Fernando Alonso was third fastest, 1.7 seconds slower than Rosberg in a turn-up for the team after a nightmare 2015 season, while Ferrari's Kimi Raikkonen was fourth in 1:24.836.
Dutch teenager Max Verstappen did the most laps of the day, completing 144 in the new Toro Rosso whose 2016 livery was unveiled earlier.
Mercedes did far more mileage than their rivals in last week's first four-day test that focused more on reliability than outright performance.
Rosberg and triple world champion Lewis Hamilton shared testing duties on Tuesday, with the Briton taking over in the afternoon for a race simulation. Together, they did 169 laps without a hitch.
Sauber had their new car running for the first time, after bringing a 2015 car to the first test, and Brazilian Felipe Nasr did 103 laps.
The season starts in Australia on March 20.