Mercedes showed off their speedy new W06 on Friday as Nico Rosberg blew away the rest of the field on the second day of the final pre-season test in Barcelona.
Using the faster soft tyres for the first time this season, Rosberg produced the fastest time through six days of testing at the Circuit de Catalunya in 1min 22.792sec.
The German's time was more than seven tenths of a second quicker than the previous best time set by Williams' Felipe Massa on Thursday.
"We are confident that we have a good car because we have a great team at the moment, but at the same time we are well aware that some of the others have been posting quick times," said Rosberg.
However, last season's runner-up in the world championship wasn't completely happy with his day's work despite the fast time in the afternoon session.
"We need to keep our heads down and keep working. Today was a difficult day because even though all the numbers looked like they were in the right place on the setup, actually the car was very far away from where it needed to be and we didn't really understand that, so the whole day we were chasing the setup and trying to find the right direction.
"Eventually in the end it was feeling more and more decent, and putting the soft tyres on gives the grip anyway so with the soft tyres it doesn't really matter what the balance is."
Williams remained the closest challengers for the world champions as Valtteri Bottas was second fastest, albeit over a second back on Rosberg.
Brazilian rookie Felipe Nasr completed another good day for Sauber as he went third quickest in a marathon 140-lap day.
There was even some encouragement for the under fire McLaren-Honda partnership as Jenson Button managed 101 laps before bringing his session to an early end when he broke down with just 15 minutes of the day left to run.
That tally equated to nearly half of the Woking-based team's testing mileage before Friday in a pre-season dogged by reliability issues and a crash suffered by two-time world champion Fernando Alonso in last weekend's second test.
"Good progress made today. It wasn't hard from yesterday, but really nice to do 101 laps," said Button.
"Reliability has been great today, really big step forward by everyone and big effort by everyone which is great to see.
"For most teams 100 laps isn't really anything, but for us it is like a 1000 laps because there is so much for us still to learn about this package."
Meanwhile, Force India finally unveiled their VJM08 car for the upcoming season and also had a positive day as Nico Hulkenberg managed 76 laps.
"It was a very good day, at least in terms of the running we managed to do. We were able to run reliably straight out of the box and that is very encouraging," said Hulkenberg.
"Obviously we still have a big task ahead of us and we are playing catch-up in certain aspects, but I am quite satisfied so far."
Results from the morning session on day two of the third pre-season test in Barcelona:
1. Nico Rosberg (GER/Mercedes) 1:22.792 (106 laps)
2. Valtteri Bottas (FIN/Williams) 1:23.995 (90 laps)
3. Felipe Nasr (BRA/Sauber) 1:24.071 (140 laps)
4. Sebastian Vettel (GER/Ferrari) 1:25.339 (142 laps)
5. Jenson Button (GBR/McLaren) 1:25.590 (101 laps)
6. Pastor Maldonado (VEN/Lotus) 1:26.705 (139 laps)
7. Max Verstappen (NED/Toro Rosso) 1:26.766 (139 laps)
8. Daniil Kyvat (RUS/Red Bull) 1:26.965 (83 laps)
9. Nico Hulkenberg (GER/Force India) 1:28.412 (76 laps)
Source: AFP
