Rosberg, winner of the Australian Grand Prix a fortnight ago, finished the day-and-night sessions at the Sakhir Circuit comfortably ahead of his Formula Oneteam-mate Lewis Hamilton.
The Silver Arrows appear to hold an ominous advantage over the rest of the field with McLaren’s Jenson Button - the surprise next-best placed driver - 1.2 seconds slower than Rosberg’s best lap of one minute and 31.001 seconds.
Indeed Rosberg’s fastest time was comfortably quicker than Hamilton’s pole lap here last year.
“I couldn’t wait to get back in the car after a great weekend in Melbourne and I felt really good out there,” said Rosberg, who has won the last four grands prix strecthing back to last season.
“It’s been an encouraging first day for the team. We were very quick on one lap and also on the longer runs, so I’m really looking forward to qualifying and the race.”
Hamilton, a winner in Bahrain for the last two seasons, added:
“There’s some work to do tonight in terms of finding a few improvements from the car and also my driving style, but I’m looking forward to getting back out there and making some more progress tomorrow.”
While it was business as usual for Mercedes, Sebastian Vettel, the driver widely expected to take the challenge to Rosberg and Hamilton in 2016, endured a troubled practice, ending the opening session in 11th before parking his Ferrari later in the day.
‘’Something has happened,’’ Vettel, sixth fastest in the second session said. ‘’I stopped the car. I lost drive.’’ A loose rear wheel nut was diagnosed as the problem.
The German’s Ferrari was then winched away on a tractor as he watched the remainder of the session trackside with the marshals.
For Fernando Alonso, a winner here on three occasions, he was forced to observe the action as a spectator in the McLaren garage after he was sidelined with a rib injury sustained in his high-speed Melbourne crash.
His participation at the next round in China remains in doubt, but his late withdrawal has paved the way for Stoffel Vandoorne, the highly-rated McLaren reserve driver, to take his grand prix bow this weekend.
The 24-year-old Belgian, last year’s GP2 champion, hastily dropped his Super Formula testing commitments in Japan to catch an overnight flight to Bahrain.
He arrived here early on Friday morning after a 13-hour flight - hardly the ideal preparation for his Formula One debut.
After an error in which he ran wide at turn 11, Vandoorne ended the first session in 18th before progressing up the leaderboard later on Friday, posting the 11th-best time. It signalled an encouraging day for McLaren, with Button the best of the rest behind the Mercedes drivers.
“There will be cars that will slip in between us tomorrow but you have got to say that it has been a good day for us, the best day for a couple of years,” Button said.
“When you are third in FP2 you have to think about getting into the final phase of qualifying. That has to be the aim.”
Max Verstappen, the ever-impressive 18-year-old, was fourth-quickest for Toro Rosso with Kimi Raikkonen fifth in his Ferrari.