Jenson Button was awarded his second straight victory for Brawn GP as the Malaysian Grand Prix had to be abandoned just after half-distance due to an enormous downpour.
Toyota's Timo Glock should have taken second thanks to a wise strategy, but fell to third behind Nick Heidfeld (BMW) on countback when the result was recalculated following the stoppage.
Jarno Trulli (Toyota) and Rubens Barrichello (Brawn) completed the top five.
The race had begun on a dry track but with very ominous clouds looming overhead.
Rosberg stormed through from fourth on the grid to lead into the first corner, with Button trying to hang on around the outside of Turn 1 only to run slightly wide and drop to fourth behind Trulli and the remarkably fast-starting Renault of Fernando Alonso.
Button swiftly re-passed Alonso, whose heavy car then bottled up most of the race of the field for much of the first stint, and then caught the two leaders, with Rosberg edging away slightly from Trulli early on.
Both Rosberg and Trulli were running lighter than the Brawn behind them, and Button made excellent use of his extra low-fuel laps to leap ahead by the time the early pitstops were complete.
The rain had still not arrived by this point, but with the clouds getting ever darker, Ferrari gambled by putting Kimi Raikkonen on extreme wet tyres on lap 18.
This proved highly premature, with Raikkonen losing up to 20 seconds per lap as the tyres struggled on the still fully-dry track.
But the rain did commence just a few laps later, prompting the whole field to dive into the pits. Everyone opted for extreme tyres bar Glock, who picked intermediates.
This proved to be an inspired choice as the rain stayed relatively light at first. The Toyota was able to take 10s per lap out of the leaders and carve through to second place, which became the lead when Button stopped for intermediates, as most of those on full wets already had done.
Once on intermediates, the Brawn took less than a lap to catch and re-pass Glock, but just as Button did so the anticipated cloudburst finally arrived, prompting yet another round of pitstops as all the intermediate runners had no choice but to stop yet again to return to full wets.
The downpour was so intense that even extreme wets were inadequate. The safety car was called on lap 31, with Button narrowly leading from Glock after both had returned to wet tyres, with the red flag following before the field had even had chance to form up in line.
After nearly an hour of waiting on an increasingly sodden grid, the officials finally admitted defeat and declared the race over.
With the results taken from the final full lap completed, Heidfeld became second ahead of Glock, despite a spin, having decided to stick with his initial choice of wet tyres.
Trulli, Barrichello and Red Bull's Mark Webber filled the rest of the top six.
Hamilton also went off the road just before the stoppage but took seventh, followed by the unfortunate Rosberg, who lost ground during the flurry of tyre changes.
Felipe Massa (Ferrari) and Nelson Piquet (Renault) completed the top ten, with Alonso falling back to 12th after a visit to the gravel in the initial shower. Raikkonen's strategic error left him only 14th.
Sebastian Vettel was the first to spin out in the rain, having made limited progress from his penalised grid spot, only getting as far as ninth before his early first pitstop.
Robert Kubica and Heikki Kovalainen had the shortest races of the all. The BMW stuttered off the grid and retired after just one slow lap, while Kovalainen spun his McLaren into the gravel only a few corners into the grand prix, meaning he has yet to complete a racing lap in 2009.
PROVISIONAL RACE RESULTS The Malaysian Grand Prix Sepang, Kuala Lumpur, Malaysia; 56 laps; 310.408km; Weather: Sunny, then wet. Classified: Pos Driver Team Time 1. Button Brawn GP (B) 55:30.622
2. Heidfeld BMW Sauber (B) + 22.722
3. Glock Toyota (B) + 23.513
4. Trulli Toyota (B) + 46.173
5. Barrichello Brawn GP (B) + 47.360
6. Webber Red Bull-Renault (B) + 52.333
7. Hamilton McLaren-Mercedes (B) + 60.733
8. Rosberg Williams-Toyota (B) + 71.576
9. Massa Ferrari (B) + 76.932
10. Bourdais Toro Rosso-Ferrari (B) + 102.164
11. Alonso Renault (B) + 109.422
12. Nakajima Williams-Toyota (B) + 116.130
13. Piquet Renault (B) + 116.731
14. Raikkonen Ferrari (B) + 142.841
15. Vettel Red Bull-Renault (B) + 1 lap
16. Buemi Toro Rosso-Ferrari (B) + 1 lap
17. Sutil Force India-Mercedes (B) + 1 lap
18. Fisichella Force India-Mercedes (B) + 2 lap
Fastest lap: Button, 1:36.641 Not classified/retirements: Driver Team On lap Kubica BMW Sauber (B) 1 Kovalainen McLaren-Mercedes (B) 0 World Championship standings, round 2: Drivers: Constructors: 1. Button 15 1. Brawn GP 25
2. Barrichello 10 2. Toyota 16.5
3. Trulli 8.5 3. BMW-Sauber 4
4. Glock 8 4. Renault 4
5. Heidfeld 4 5. Williams-Toyota 3.5
6. Alonso 4 6. Toro Rosso-Ferrari 3
7. Rosberg 3.5 7. Red Bull-Renault 1.5
8. Buemi 2 8. McLaren-Mercedes 1
9. Webber 1.5
10. Hamilton 1
11. Bourdais 1
All timing unofficial