Elsewhere there was encouragement for McLaren, as Jenson Button completed more than 100 laps on the team's most successful day to date, and Force India, who rolled out their new car for the first time and clocked up close to 80 laps across the afternoon.
But it was Mercedes who stole the show. Despite being the early favourites given their dominance in 2014, the Silver Arrows had not troubled the top of the timesheets over the opening nine days of testing. That changed, however, as the team bolted on Pirelli's soft compound for the first time this year, allowing Rosberg to clock a blistering 1m 22.792s on his first run.
The German came close to bettering that on subsequent runs, but it was ultimately good enough to keep him comfortably top, more than a second clear of the field - and more than 2.5s faster than Lewis Hamilton's 2014 pole time at the Circuit de Barcelona-Catalunya.
"I'm pleased with my two laps at the end - I nailed them pretty well and they were both identical, so a good ending to the day," said Rosberg.
"We're confident that we have a good car and I'm starting to feel good in it. At the same time, of course, we're well aware that the others have been posting some quick times and we can take nothing for granted."
Williams locked out second in Mercedes' wake courtesy of Valtteri Bottas, who took over from day-one pacesetter Felipe Massa, with Sauber's Felipe Nasr fractions of a second down the road in third.
Ferrari's Sebastian Vettel was fourth, albeit 2.5s off Rosberg's pace, while Button earned McLaren fifth place, a welcome boost to add to their lap tally.
“We’ve still got a lot of work still do to, but we had a good day today,” said Button, who conducted front-wing work in the morning and long runs in the afternoon.
“It’s never easy when you’ve missed out on so much testing, but we’re making good progress, and we got lots of really useful information today.
“We know we’re not the quickest guys out there, but you’ll certainly see a lot of improvement by the time we get to Australia.”
Lotus's Pastor Maldonado shaded Toro Rosso's Max Verstappen and Red Bull's Daniil Kvyat for eighth, with Nico Hulkenberg rounding out the day's runners in ninth for Force India.
“It was a very good day, at least in terms of the running we managed to do," said Hulkenberg following his initial stint in the new VJM08.
"We were able to run reliably straight out of the box and that is very encouraging. The boys and girls at the factory and here at the track have been pushing really hard in the last couple of weeks and to do 77 laps in little more than one afternoon is a remarkable job by the team.
"Obviously we still have a big task ahead of us and we are playing catch-up in certain aspects, but I am quite satisfied so far."
Verstappen was one of the day's busiest men with 139 laps, although his running was cut slightly short by an electrical problem on the STR10.
"Another good day as we did a lot of laps," Verstappen said. "From my perspective, the most important part was this afternoon's race distance, when the pace looked very promising. It was my first time driving the car with the latest updates and they seemed to be working very well and I felt very comfortable with it."
Button also hit issues late on when he stopped out on track between Turns 7 and 8, bringing out the red flags for only the second time of the day. Kvyat had prompted the first stoppage when he was instructed to halt out on track as a precaution after Red Bull discovered a hydraulic sensor issue.
Unofficial Friday testing times from Barcelona:
1. Nico Rosberg, Mercedes, 1m 22.792s, 106 laps
2. Valtteri Bottas, Williams, 1m 23.995s, 90 laps
3. Felipe Nasr, Sauber, 1m 24.071s, 141 laps
4. Sebastian Vettel, Ferrari, 1m 25.339s, 143 laps
5. Jenson Button, McLaren, 1m 25.590s, 101 laps
6. Pastor Maldonado, Lotus, 1m 26.705s, 140 laps
7. Max Verstappen, Toro Rosso, 1m 26.766s, 139 laps
8. Daniil Kvyat, Red Bull, 1m 26.965s, 84 laps
9. Nico Hulkenberg, Force India, 1m 28.412s, 77 laps