Nico Rosberg praised Mercedes after his first full day in the W07 Hybrid but maintains that the squad needs to be innovative in order to stay ahead of the opposition.
Rosberg's Mercedes featured new winglets on the sidepod and he placed fourth after completing 172 laps, having set his best time on Medium tyres, compared to the Ultra Softs which adorned Sebastian Vettel's Ferrari.
"That was an F1 Marathon today! It's great to see that we are already able to do a lot of mileage - and we didn't have a single problem either, which is really incredible," he said.
"I'm very happy and proud of what the boys and girls at Brackley and Brixworth have done over the winter. I already feel very comfortable in the car.
"It's a big positive to see everyone pushing so hard in all areas - bringing new developments like the floor we saw today, for example.
"The opposition are chasing us hard and we need to keep being innovative to stay ahead. At this time it's not possible to say how quick we are compared to the others.
"We will maybe know more at the end of next week - but only Melbourne will give the full picture."