The German hit the front after bolting on Pirelli's yellow-marked soft compound for the first time in 2016 - the Silver Arrows having only run the mediums and hards during the opening test last week. The result was a lap of 1m 23.022s - the second fastest lap so far in testing, and nearly two seconds faster than anyone else managed on the morning.
Kimi Raikkonen kept Ferrari within touching distance at least - he set a 1m 24.836s on mediums, which had been quickest until Rosberg's surge.
There was encouragement for McLaren and Fernando Alonso meanwhile, as the Spaniard - restricted to just three laps on the final day of last week's test - racked up 50 laps and set the third fastest time. He did use the softs for his fastest run however, and wound up 2.2s down on Rosberg.
Alonso also triggered the red flags when he stopped at the end of the pit lane right at the end of the morning session.
Behind Alonso, there was little to separate Williams' Valtteri Bottas and the newly-liveried Toro Rosso of Max Verstappen in the hunt for fourth. Bottas and Verstappen - on 58 and 59 laps respectively - were also the morning's busiest runners except Rosberg, who clocked up 82 on another day of mammoth mileage for Mercedes.
Behind them Nico Hulkenberg put Force India sixth ahead of Red Bull's Daniil Kvyat - whose morning was compromised by a small brake fire - Haas's Esteban Gutierrez and the Sauber of Felipe Nasr.
The Brazilian had a slow start to the morning as Sauber focused on installation runs in the new C35-Ferrari, which hit the track for the first time.
Nasr still managed more laps than Manor's Rio Haryanto however. The Indonesian completed just one lap before a leak kept him stuck in the garage, with the team appearing to fix the issue just before the hour-long break for lunch.
|1||Nico Rosberg||Mercedes F1 W07 Hybrid||1:23.022||82|
|2||Kimi Raikkonen||Ferrari SF16-H||1:24.836||1.814||42|
|3||Fernando Alonso||McLaren-Honda MP4-31||1:25.252||2.230||50|
|4||Valtteri Bottas||Williams-Mercedes FW38||1:25.255||2.233||58|
|5||Max Verstappen||Toro Rosso-Ferrari STR11||1:25.356||2.334||59|
|6||Nico Hulkenberg||Force India-Mercedes VJM09||1:25.954||2.932||50|
|7||Daniil Kvyat||Red Bull Racing-TAG Heuer RB12||1:26.227||3.205||22|
|8||Esteban Gutierrez||Haas-Ferrari VF-16||1:26.661||3.639||23|
|9||Felipe Nasr||Sauber-Ferrari C35||1:26.923||3.901||42|
|10||Kevin Magnussen||Renault R.S.16||1:26.981||3.959||54|
|11||Rio Haryanto||Manor Racing-Ferrari MRT05||-||2|