Mercedes Executive Director Toto Wolff claims the Brackley outfit heads into the new season with 'humility' amidst claims that they start the year as favourites to win the Constructors' Championship.
Speaking to German news agency DPA, the Austrian admitted that they "cautiously optimistic" after performing well in pre-season testing so far.
"Humility is the word that comes to mind. There have been many 'test world champions' in the past," Wolff told DPA.
"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."
Wolff added that reigning Constructors' Champions Red Bull cannot be discounted, despite their struggles at Jerez and Sakhir.
"This team (Red Bull) has won the world championship four times and has one of the best drivers in the field," he continued.
Despite topping the time sheets in Bahrain last week, Wolff admitted that their winter testing hasn't been perfect.
"The issue of reliability is far from solved," the 42-year-old added.
"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."
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