BARCELONA, Spain (AP) The usually faultless Mercedes experienced a rare blip Friday during an otherwise impressive eight days of preseason testing that showed the car's outstanding reliability ahead of the season-opening Australian Grand Prix.
Over the testing, Mercedes logged a mind-boggling 1,294 laps of the 4.655-kilometer (2.9-mile) Spanish circuit encircled by rolling hills - equivalent to nearly 20 Grand Prix distances.
''It's been an amazing two weeks for the team,'' Hamilton said. ''The guys have done an amazing job to get the car to where it is especially given the fact we've done more mileage in pre-season than we've ever done before.''
The blip itself was small enough not to cause any concern at Mercedes, and happened when Lewis Hamilton stalled the W07 car at the pit lane exit shortly before the end of the morning session.
While four-time F1 champion Sebastian Vettel clocked a leading time of 1 minute, 22.852 seconds in his smooth-running Ferrari, Hamilton's car had a transmission problem and went back to the garage for further assessment.
''I'm kind of glad I broke the car at the end of my final run today because it's better to have found something now than in Melbourne,'' Hamilton said. ''I'm looking forward to putting the car through its paces at the first race.''
The problem delayed teammate Nico Rosberg's afternoon stint by a couple of hours, and he finished the day in the unusual position of having the slowest time - although he was on medium tires and not testing out his speed.
''This afternoon was probably the best the car has felt for me all the way through winter testing, so that was a really positive way to end the week,'' Rosberg said. ''Of course, we haven't seen the full picture yet in terms of (speed), and everybody will be excited to find that out in Australia.''
Vettel's mark stood all day.
Ferrari has shown good speed in testing and the German driver was .282 seconds quicker than Spanish driver Carlos Sainz, who underlined the reliability of the Toro Rosso car with 133 laps.
Brazilian Felipe Massa had the third best time for Williams ahead of Mexican Sergio Perez's Force India and Hamilton.
But Hamilton will doubtless feel frustrated that he did not get a chance to try out the new ultra-soft tires ahead of the March 20 GP in Melbourne.
Hamilton, who was on medium tires when the incident happened, had spoken enthusiastically on Thursday of testing out the ultra-softs and putting the car on a ''knife edge.''
He was not granted his wish.
Instead, the red flag ended his day and Hamilton was more than a second behind Vettel's leading time on super-soft tires before handing over to Rosberg.
However, it's unrealistic to directly compare Ferrari and Mercedes for speed, considering Mercedes used medium tires for the entire testing program, whereas Ferrari was regularly trying out softer tires.
''We have to see if this car can be a winner, but I think that the first impression was good and everything we have built is positive,'' Vettel said. ''I am happy with the feeling in the car. And we are happy with the step we have made. In terms of reliability, though, Mercedes seems to be very strong, irrespective of the issue they had today.''
Like his teammate Kimi Raikkonen the day before, Vettel also did an installation lap with a prototype of the new head protection device fitted to his Ferrari SF16-H.
The sport is looking at ways of improving cockpit security to leave drivers less exposed to the risk of head injuries after French driver Jules Bianchi and British IndyCar driver Justin Wilson died last year.
The ''halo'' design works by forming a kind of semi-circular barrier around the driver's head and is the concept most favored as it offers protection against flying debris without completely closing the cockpit.
Although Vettel thinks it needs improving, visibility does not seem to be a major concern.
''To go around it's OK, you can see what you need to see. You can improve the aesthetics,'' Vettel said. ''In principle I agree it doesn't look very nice. It's not a picture we're used to in Formula One. But it helps increasing the safety and saving lives.''
Rosberg also backed the decision to use the halo system.
Meanwhile, French driver Romain Grosjean, who is driving for American-led newcomer Haas, was relieved to have a stress-free day after stopping three times on Thursday because of braking issues.
''It felt much better, it was a good relief for everyone,'' Grosjean said. ''The guys worked hard, all night long to solve the issue.''
Grosjean handed over to teammate Esteban Gutierrez for the last hour, giving the Mexican driver some more lap time after he spent all day Wednesday stuck in the garage.