Lewis Hamilton was fastest, Carlos Sainz had more bad luck, McLaren changed their engines while Ferrari were pretty low-key in FP1 at the Italian GP.
It was another commanding session for Mercedes as Hamilton posted a 1:24.670 with Rosberg 0.463s behind his team-mate while Sebastian Vettel was best of the rest, but the four-time World Champion was more than 1.5s adrift in third place.
Hamilton and Rosberg, of course, have new-spec power units at their disposal this weekend after Merc opted to use their remaining seven in-season engine development tokens ahead of the race.
Mercedes' biggest challengers this year, Ferrari, had a quiet day as Vettel and Kimi Raikkonen, who was P6, completed only 33 laps between them compared to the 25 laps that Hamilton did on his own.
Ferrari are also using "a few" tokens for their home race, but it looks like the fight for the final podium place could be interesting as the Merc-powered Force India were right up there with Nico Hulkenberg and Sergio Perez fifth and sixth respectively.
Daniel Ricciardo, Felipe Massa, Valtteri Bottas and Pastor Maldonado completed the top 10.
A bit further down the track, the unfortunate Sainz once again had problems with his Toro Rosso as the car went sliding off at the final corner and ended up in the gravel trap.
The Spaniard, who has failed to see the chequered flag in his last four races, completed only eight laps and admitted he didn't know what was wrong with the car.
Fernando Alonso and Jenson Button also had limited running as they completed 23 laps between them with McLaren opting to change their Internal Combustion Engines (ICE) ahead of FP2. It will result in a five-place grid penalty for Button while Alonso will drop 10 places. Of course, they will simply be moved to the back of the grid.
1 Lewis Hamilton Mercedes 1:24.670 25 laps
2 Nico Rosberg Mercedes 1:25.133 0.463 22 laps
3 Sebastian Vettel Ferrari 1:26.258 1.588 17 laps
4 Nico Hulkenberg Force India 1:26.612 1.942 19 laps
5 Sergio Perez Force India 1:26.730 2.060 29 laps
6 Kimi Raikkonen Ferrari 1:26.783 2.113 16 laps
7 Daniel Ricciardo Red Bull 1:26.922 2.252 27 laps
8 Felipe Massa Williams 1:26.936 2.266 22 laps
9 Valtteri Bottas Williams 1:27.075 2.405 25 laps
10 Pastor Maldonado Lotus 1:27.118 2.448 28 laps
11 Felipe Nasr Sauber 1:27.232 2.562 21 laps
12 Daniil Kvyat Red Bull 1:27.275 2.605 29 laps
13 Marcus Ericsson Sauber 1:27.454 2.784 17 laps
14 Max Verstappen Toro Rosso 1:27.591 2.921 27 laps
15 Jolyon Palmer Lotus 1:27.669 2.999 28 laps
16 Carlos Sainz Toro Rosso 1:27.907 3.237 9 laps
17 Fernando Alonso McLaren 1:28.023 3.353 11 laps
18 Jenson Button McLaren 1:28.423 3.753 12 laps
19 Will Stevens Manor 1:29.853 5.183 22 laps
20 Roberto Merhi Manor 1:29.911 5.241 12 laps