Lewis Hamilton admits he has been stunned by the "incredible" form shown by Mercedes so far this pre-season.
The German manufacturer completed a total of 675 laps over four days of testing in Barcelona, which was 228 more than next-best Toro Rosso.
While they did not top the timesheets in any of the days, Mercedes shocked their rivals with their incredible consistency.
"I've never seen anything like it. Just the strength of this car, it just keeps going," reigning world champion Hamilton said.
"After winning two championships, it's so easy to lose focus - but everyone has done an even better job.
"It's just been an incredible weekend for testing for us. Before we came here they were talking about doing 800km (497 miles) a day and I thought that was crazy because I've never done that in the past.
"There are no breakdowns and a lot of reliability.
"The car looks fantastic. Obviously we don't know the pace of the car just yet but what we can say is that it's gone the distance and that's half the battle."
Mercedes also debuted a new nose design on Thursday which included a number of unusual aerodynamic features.
Hamilton added: "The front wing is an improvement, which is good. It's always about small increments and that's what it's brought."
Testing resumes for another four days beginning on March 1 at the Circuit de Barcelona-Catalunya.