Last year's champion constructor had been content to allow world champion Lewis Hamilton and Rosberg rack up the laps on medium compound tires but in the afternoon session at the Circuit de Catalunya, the German star was given his head.
He lapped in one minute 22.792 seconds, a full 0.7 seconds quicker than the next best of Felipe Massa in the Williams.
It was also over a second better than his nearest rival on the day, Massa's teammate Valtteri Bottas of Finland.
With the opening grand prix of the season in just over a fortnight in Australia, Mercedes has laid down the groundwork to repeat its 2014 triumphs where it won 16 of the 19 races.
Not least because it has run for over 1,000 laps in the three pre-season tests at Jerez and Barcelona, which end Sunday.
Red Bull -- with Daniel Ricciardo scoring three impressive wins -- was its closest rival but the Australian's best time set Thursday on soft tires was one minute 24.574, considerably off the Mercedes pace.
Ferrari, with Sebastian Vettel new to its ranks, has showed signs of improvement and the four-time world champion got a best of the day 143 laps under his belt.
There was better news as well for the CNN-sponsored McLaren-Honda team which suffered a further setback when two-time world champion Fernando Alonso was hospitalized after a crash Sunday at the same circuit.
The Spaniard left hospital Wednesday and has posted a YouTube message saying he was "completely fine" and would be taking his place on the grid for the season-opener in Melbourne.
In his absence, teammate Jenson Button has been playing desperate catch up with their rivals on the grid.
The 2009 world champion totaled 101 laps in the MP4-30, which has been plagued with teething problems, but the session ended on a low note when it stopped on the track to cause a red flag.
Force India also finally unveiled its VJM08 car for the new campaign and Nico Hulkenberg managed 76 laps.
Marussia to return?
In other F1 news, Manor Marussia has been included on the official entry list for the new season.
Financially-troubled Marussia came out of administration this month after failing to take part in the final three races of last season when its cash ran out.
It was also still reeling from the horrific crash of its star driver Jules Bianchi at the Japanese Grand Prix.
England's Will Stevens has been confirmed as its first driver and is set to take his place on the grid at Albert Park on March 15.
Stevens raced in last season's final race in Abu Dhabi for Caterham, another struggling team, which has been scrapped from the 2015 list after failing to find new funding.