Mercedes may have covered the most miles at Jerez but Toto Wolff says they are "some way" from reaching the full potential of the new V6 engine.
Mercedes drivers Lewis Hamilton and Nico Rosberg had the better of the running at the opening pre-season test with the duo clocking 309 laps, 58 more than the second best team, Ferrari.
That, coupled with some solid pace, has led to claims that Mercedes could be the favourite to take this year's titles.
However, executive director Wolff has been quick to downplay the favourite tage.
"If you look back through past seasons, the favourites after the first few days of testing are not always in the same position at the first race," he said.
"I think we have to have a certain degree of caution, especially at this early stage.
"A few different teams have looked strong so far in different ways but we won't know the full story for a while yet, not even after the first couple of races I wouldn't say."
The Austrian added that Mercedes have yet to get the most out of the new engine.
"I would say cautiously satisfied," he said when asked about the progress.
"The power unit and all the associated systems are very new so making sure they funtion properly and are reliable has been the focus.
"We are some way from what we expect to be our full potential and there are a few things not quite functioning as we would like, however we are proud of the integrated concept produced by the teams at Brackley and Brixworth."
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