Amid claims Mercedes is the overwhelming 2014 favourite, boss Toto Wolff says the Brackley team is heading into the season with "humility".

Admitting he is "cautiously optimistic", Wolff told the German news agency DPA he is sure that if Mercedes has a big advantage now, it will not last long.

"We are very aware that Ferrari and the Renault teams will come back. It is only a matter of time and I don't doubt that for a second," said Wolff.

He said world champions Red Bull and Sebastian Vettel in particular - despite their huge problems with the Renault-powered RB10 - cannot be ruled out.

"This team (Red Bull) has won the world championship four times and has one of the best drivers in the field," said Wolff.

But he agreed that, in total contrast, Mercedes has had a fairly smooth winter despite the massive challenge of the all-new 2014 rules.

Wolff insisted: "Humility is the word that comes to mind. There have been many 'test world champions' in the past."

And he said even the dominant Mercedes W05 has not been perfect.

"The issue of reliability is far from solved," said Wolff. "I assume that the first issue has to be getting to the end of the race.

"We have had some good tests but we have no reason to fall into jubilation," the 42-year-old added.

(GMM)