|Rosberg tops the standings ahead of Saturday's practice run [GALLO/GETTY]
Nico Rosberg was fastest in Friday's practice for the season-opening Bahrain Grand Prix, staying ahead of teammate Michael Schumacher who finished third to put Mercedes at the front of the Formula One grid.
Rosberg set the quickest time of 1 minute, 55.409 seconds to lead Lewis Hamilton of McLaren and Schumacher by just under half a second after two sessions.
Compatriot Adrian Sutil had stolen some of the returning seven-time champion Schumacher's limelight in the first session with the quickest time for Mercedes-powered Force India, but Rosberg went faster in the afternoon.
Hamilton was second, running faster than team mate and reigning champion Jenson Button who had been ahead in the morning.
Mercedes-engined cars filled the top four slots.
Schumacher said he needed to raise his game and shake off the rust in time for Saturday's qualifying.
"It's a bit too much for my standards, so I gotta raise my game a bit and I'm sure I'm going to,'' said the 41-year-old German, who skidded off the track at one point during the first session.
"It has been fun but I certainly feel a little bit rusty, especially on the (qualifying lap) issue. I'll just use the rest of the weekend to tweak out a few little things.''
Spain's double world champion Fernando Alonso limbered up for his Ferrari race debut with the second best time in the first session and then ninth after lunch.
"I have no idea,'' Alonso answered when asked if his Ferrari was capable of putting him on pole position for Sunday's race.
"It's still completely unknown for everybody. The four big teams, any of them can be quick tomorrow and be on pole.''
While practice times can be misleading, with drivers trying out different set-ups and running with varying fuel loads, all eyes were on Schumacher's pace after an absence of three seasons.
With Mercedes returning as a works team for the first time since 1955 after taking over champions Brawn, the former Ferrari great was 10th in the morning and more than 0.4 seconds slower than Rosberg.
Lotus, an illustrious name returning after a 16-year absence were the best of the three all-new entrants although they were an age off the pace.
Finland's Heikki Kovalainen lapped 18th fastest, 5.464 seconds slower than Rosberg, with Italian Jarno Trulli 19th.
Virgin Racing's Timo Glock was 6.6 seconds slower than the fastest car while Brazilian Bruno Senna was 11.5 seconds away from Rosberg's time.
1 Rosberg (GER) Mercedes 1:55.409
2 Hamilton (GBR) McLaren 1:55.854
3 Schumacher (GER) Mercedes 1:55.903
4 Button (GBR) McLaren 1:56.076
5 Vettel (GER) RedBull-Renault 1:56.459
6 Huelkenberg (GER) Williams-Cos 1:56.501
7 Massa (BRA) Ferrari 1:56.555
8 Petrov (RUS) Renault 1:56.750
9 Alonso (ESP) Ferrari 1:57.140
10 Rosa (ESP) BMW Sauber-Ferrari 1:57.255
11 Kobayashi (JAP) BMW Sau-Fer 1:57.352
12 Sutil (GER) Force India-Mercedes 1:57.361
13 Barrichello (BRA) Williams-Cos 1:57.452
14 Liuzzi (ITA) Force India-Mercedes 1:57.833
15 Kubica (POL) Renault 1:58.155
16 Alguersuari (ESP) Toro Ros-Fer 1:59.799
17 Webber (AUS) RedBull-Renault 2:00.444
18 Kovalainen (FIN) Lotus-Cosworth 2:00.873
19 Trulli (ITA) Lotus-Cosworth 2:00.990
20 Glock (GER) Virgin-Cosworth 2:02.037
21 Di Grassi (BRA) Virgin-Cosworth 2:02.188
22 Senna (BRA) HRT-Cosworth 2:06.968
23 Buemi (SUI) Toro Rosso-Ferrari
24 Chandhok (IND) HRT-Cosworth
Hot temperatures at the desert track came into play and accelerated tyre degradation.
Drivers will need to carefully manage tyres on heavy cars as a result of new rule changes that did away with refuelling in the upcoming season.
"It's a bit of a dog to drive it on a heavy fuel tank,'' Hamilton said of the heavier set-up.
"It's like driving a bus. (The car) doesn't stop as well.''
The drivers were also unhappy about the new-look track which has added eight new corners to slow the cars.
"It's hard to concentrate on the next corner with your head bouncing all over the place. It's tricky,'' Button said of the new section.
"It's a good place to lose a lot of time so you have to be careful. It doesn't help overtaking in any way.''
Button said he was "reasonably happy'' with his car and his new start at McLaren after winning the title with Brawn GP - now Mercedes - last year.
"I've chosen my new challenge and that is to race here. I think I needed that challenge after winning the championship,'' the Briton said.
"It's not that I'm not hungry, I'm so hungry.''
While the top teams prospered, the rookie teams continued to struggle at the back of the grid with Bruno Senna losing a piece of his rear tyre on the final lap in his Hispania Racing car.
Senna, nephew of three-time champion Ayrton Senna, managed 20 laps over the two sessions with a best time of 2:06.968 - nearly 12 seconds off the pace.
With the HRT (Hispania) team unable to test the car before arriving in Bahrain, the return of the Senna name to a grand prix weekend for the first time since the death of Ayrton at Imola in 1994 was limited to what amounted to a shakedown.
Indian rookie team mate Karun Chandhok did not even manage to get out on track, forced to sit out both sessions while mechanics worked on his car.
"We have had a lot of problems, we can't seem to get the clutch and gearbox working," he said.
"It's such a shame for the mechanics, they have worked all night without any sleep - so I feel for them. It's some sort of electronic or hydraulic issue, it's not mechanical... it's a very difficult first day in F1 for me."