BARCELONA, Spain (AP) Mercedes showed a glimpse of the speed that so terrifies its rivals as Nico Rosberg posted the fastest time Tuesday in preseason testing for the Formula One season, while world champion Lewis Hamilton used his time on the track for a race simulation.
The two teammates completed 675 laps in last week's first preseason test, confirming expectations that the Silver Arrows will be hugely reliable again when the new season starts on March 20.
Ferrari was quickest over all four days of testing last week, but they were illusory gains until Mercedes showed its speed this week.
With Mercedes using soft tires for the first time - having tested slower medium tires last time - Rosberg sped around in 1 minute, 23.022 seconds.
''Of course, it is a great feeling to know that our level is exceptionally high right now and the team is working sensationally,'' Rosberg said, before sounding the obligatory note of caution. ''We have to keep our heads down and do our homework. We do not want any bad surprises.''
Rosberg and Hamilton will keep alternating shifts until testing ends Friday.
''The good thing is that I will be in the car every day,'' Rosberg said. ''So I have time to analyze, to look at the data and things that I can do better.''
Finnish driver Valtteri Bottas was second fastest - .207 behind Rosberg - and Hamilton was sixth.
With the car's reliability not likely to be an issue leading up to the Australian GP in Melbourne, racking up laps is not Mercedes' top priority.
But Rosberg still managed 82 laps before lunch, while Hamilton - arriving at the track accompanied by his two dogs - did 90 after the resumption under blue skies and in mild conditions at the Catalunya track just outside of Barcelona.
Hamilton ran a simulation run of the Spanish GP - which will be held here on May 15 - using both sets of medium compounds and saving the quick softs for another day. Having won back-to-back drivers' and constructors' titles, Mercedes is under no pressure to prove its speed.
''Where we are now it looks like it's going to be quite tricky for anyone to challenge them at the beginning of the season,'' said Bottas, who logged 123 laps for Williams. ''We are doing our best. We're really trying to chase them.''
There was an encouraging performance for McLaren with Fernando Alonso third fastest and managing 93 laps.
Given McLaren's problems throughout last season with Honda engines, this will please McLaren's technical staff as much as two-time F1 champion Alonso - who recently gave an interview to Spanish radio insisting he can still win another F1 title.
The red flag came out early into the afternoon after Kimi Raikkonen's Ferrari pulled up with a gearbox problem that kept his car in the garage for a couple of hours. However, he did manage the day's fourth quickest time ahead of Red Bull driver Daniil Kvyat, who had some rear brake issues.
''It is normal to have some little issues during testing, but I am not concerned as we have time to fix it,'' said Raikkonen, the 2007 F1 champion. ''So far, the car feels good.''
In the morning, Manor driver Rio Haryanto went back into the garage because of an oil leak, but the F1 newcomer from Indonesia completed 45 laps.
Haryanto was the slowest on the day, nearly five seconds behind Rosberg. Mexican Esteban Gutierrez, who did 23 laps before a fuel system issue sent him back to the Haas garage, was the second-slowest. Haas is the first American-led team in F1 since 1986.
''It's not ideal,'' Gutierrez said. ''You can have these days when the whole program is interrupted. We are a new group.''
Before the session, Toro Rosso unveiled its dark blue livery for the new season, with the young and highly-talented duo of Max Verstappen and Carlos Sainz promising much after successful debut seasons. The Italian team has switched from Renault to 2015 Ferrari engines.
Last week, Verstappen and Sainz notched 447 laps - second only to Mercedes - and Verstappen showed his endurance in recording 144 laps for the day's highest total. The 18-year-old Dutch driver was the seventh quickest.
Force India's Nico Hulkenberg was eighth; Sauber's Felipe Nasr was ninth and Renault's Kevin Magnussen 10th, with all managing more than 100 laps.