Rosberg had recorded three laps good enough to put him quickest of all, until Barrichello posted a late flier, as the three teams whose diffusers are under protest dominated the top of the timesheets.
In a busy second session in terms of laps, if not incidents, as the teams began to work their new cars into a groove, Jarno Trulli and Toyota team-mate Timo Glock traded times initially until Rosberg moved to the top after half an hour with a 1:26.293 lap.
He then fired in a 1:26.142 before pitting, bolting on a new set of tyres and recording a 1:26.053 - comfortably the fastest time of the weekend so far.
Trulli, who amassed a whopping 42-lap total remained ended up third, 0.297 seconds back, with Mark Webber, the only man to brake into the 'diffuser gang' - producing an extraordinary lap in his Red Bull that put him fourth fastest.
His team-mate Sebastian Vettel spun trying to match the Australian after putting a wheel on the grass under braking for Turn 4. For the second time today, the German was the only man not to complete the session.
Jenson Button was fifth fastest ahead of Timo Glock, as second to sixth was covered by two tenths of a second.
Kazuki Nakajima was seventh in the second Williams, with Vettel eighth.
Force India showed well in the afternoon session. The Mercedes-powered cars of Adrian Sutil and Giancarlo Fisichella ran second and third early in the session. The German ended it ninth with his veteran team-mate 13th.
Felipe Massa, who looked to be struggling under braking in the Ferrari, was tenth, a second off the ultimate pace and a tenth faster than team-mate Kimi Raikkonen. Both drivers running off track on more than one occasion.
In fact, not one of the three world champions in the field could break into the top ten.Fernando Alonso could manage no better than 12th for Renault, while the reigning title holder Lewis Hamilton complained over the radio of severe bottoming out as he ended the session 18th, one place behind his McLaren team-mate Heikki Kovalainen.
Pos Driver Team Time Laps
1. Rosberg Williams-Toyota (B) 1:26.053 36
2. Barrichello Brawn-Mercedes (B) 1:26.157 + 0.104 38
3. Trulli Toyota (B) 1:26.350 + 0.297 42
4. Webber Red Bull-Renault (B) 1:26.370 + 0.317 30
5. Button Brawn-Mercedes (B) 1:26.374 + 0.321 38
6. Glock Toyota (B) 1:26.443 + 0.390 42
7. Nakajima Williams-Toyota (B) 1:26.560 + 0.507 33
8. Vettel Red Bull-Renault (B) 1:26.740 + 0.687 19
9. Sutil Force India-Mercedes (B) 1:27.040 + 0.987 29
10. Massa Ferrari (B) 1:27.064 + 1.011 35
11. Raikkonen Ferrari (B) 1:27.204 + 1.151 32
12. Alonso Renault (B) 1:27.232 + 1.179 28
13. Fisichella Force India-Mercedes (B) 1:27.282 + 1.229 32
14. Heidfeld BMW-Sauber (B) 1:27.317 + 1.264 34
15. Kubica BMW-Sauber (B) 1:27.398 + 1.345 36
16. Bourdais Toro Rosso-Ferrari (B) 1:27.479 + 1.426 36
17. Kovalainen McLaren-Mercedes (B) 1:27.802 + 1.749 35
18. Hamilton McLaren-Mercedes (B) 1:27.813 + 1.760 31
19. Piquet Renault (B) 1:27.828 + 1.775 35
20. Buemi Toro Rosso-Ferrari (B) 1:28.076 + 2.023 33