Jerez F1 testing: Mercedes makes its best start yet - Toto Wolff

Nico Rosberg completed 157 laps - 84 more than any of Mercedes' rivals - on the way to the third-fastest time on Sunday.

Asked if he could have predicted such a high mileage tally, Wolff joked: "You've discovered our secret project: it's Le Mans, we're doing endurance testing.

"No, I don't want to sound arrogant or over-confident. We need to stay humble.

"It was a really good first day. For me it was the best one I've seen in terms of mileage and reliability.

"We didn't push the limits in terms of the car and its systems.

"But for a first day we were very solid. Nico says it's one of the best first days he's had.

"Does it mean a lot? No. But it's better to have a good first day than a bad one."

Rosberg also urged against over-confidence despite Mercedes' strong performance.

"Are we ahead of the game? No. But we got off to a good start for ourselves," he said.

"Not comparing with anyone else, because it's not possible to compare at the moment.

"We can only judge in terms of what we wanted to achieve today, and we managed to achieve that.

"The laps that we wanted to do, the pitstop practice at the end, for us it was a good start."

The 2014 F1 runner-up added that the new W06 felt promising in its handling as well as its reliability.

"I can just say that I have a good feeling," said Rosberg.

"There is no reason for it, there are no numbers for it, it's just a feeling.

"It's been reliable, no major problems, and the car balance feels more or less decent.

"Of course it's not perfect, and there's still some work to do, and I look forward to that work, starting tonight."