"When you think about where they were three weeks ago," said Mercedes' Toto Wolff, "then we need to be really aware of what they are capable of.
"We are yet to see a reliable Vettel, and when that happens he'll be really fast," the German told Finland's Turun Sanomat newspaper.
Williams is currently regarded as the second-fastest team behind Mercedes, but technical boss Pat Symonds is expecting a tough fight in 2014.
"Look at how quickly Renault has caught up," he told Germany's Auto Motor und Sport. "Three weeks ago they were nowhere, suddenly they have a car that can be second."
Indeed, a lot of the responsibility for Red Bull's issues is in engine supplier Renault's court.
"It is obvious that we are slower on the straights than Mercedes," the French marque's Remi Taffin is quoted by Italiaracing.
"I don't know if it's a second, and it's definitely more than a tenth, but there are no unsolvable problems," he insisted.
Also chasing Mercedes hard are McLaren and Ferrari.
"We believe that their (Mercedes') advantage is between five tenths and three quarters of a second," McLaren team boss Eric Boullier is quoted by France's Auto Hebdo.
"From our side we wanted above all to develop a reliable car, as we think it is essential to win a lot of points early in the season.
"Then we will have a very aggressive development," said Boullier.
"We need to catch up with Mercedes but also other teams that will be successful when their engine is as well."
Ferrari agrees that, despite a disappointing opening weekend in Australia, nothing is lost in terms of the overall battle for the 2014 title.
"I honestly believe that there is time for this team to fight for this world championship," boss Stefano Domenicali is quoted by the Spanish sports newspaper AS.
"Sure, it will not be easy, but of course there is time and we have the resources to do everything possible to bring it about."