Vettel admits Rosberg's time on soft tyres was "awesome" as World Champions show their hand.
After a winter of playing it cool, Mercedes finally unleashed its true pace for everyone to see on Friday.
Nico Rosberg's fastest laptime in the previous Barcelona test, which was only two tenths off the best time but set on tyres two steps harder than the pacesetting Lotus of Romain Grosjean, was an indicator of a speed advantage.
However, Rosberg's 1:22.792 yesterday, set on soft tyres, was over 1.2 seconds quicker than the Williams of Valtteri Bottas on similar rubber, and the gulf in class was plain for all to see.
"Everyone knew they were quick," said Bottas, "but they were very quick. But that's how it is."
To put it into context for the test as a whole, Rosberg's lap was 0.708s quicker than Felipe Massa managed for Williams on the opening day on Thursday.
Vettel not surprised by Merc advantage
Ferrari's Sebastian Vettel said Rosberg's laptime was "awesome".
As for Mercedes picking up in Melbourne where it left in Abu Dhabi three months ago, Vettel said: "Anything else would be a huge surprise.
"What will happen behind them is less clear."
Mercedes team principal Toto Wolff denied that the reigning 2014 champions have been hiding the potential of the new W06 car until now.
"Our programme was planned from the beginning," he said.
"Other may have done it a bit differently to us. We estimate that we should still be ahead."