MANAMA (AFP) - Germany's Nico Rosberg registered the fastest time in his Mercedes in Saturday's pre-season testing session at Bahrain's Sakhir circuit as world champions Red Bull endured another frustrating day.

Rosberg managed to go a second faster than his teammate Lewis Hamilton a day earlier, outpacing Jenson Button's Mclaren and Kimi Raikkonen in a Ferrari at the end of an encouraging week for Mercedes.

"This morning we were able to try some qualifying practice runs which felt good, meaning I could attack the lap a bit as I found a good balance," Rosberg told the sport's official website, www.F1.com.

After a late crash which brought his day to a premature end Raikkonen, said: "Hitting the barrier? I spun on the kerb at the exit of turn 4, these things can happen.

"Luckily it was right at the end of the day, I'm only sorry because it means the team now has more work to do."

Brazilian Felipe Nasr, a new test driver with the Williams team, took to the wheel for the first time and was fourth-quickest.

"There were so many things going through my head before sitting in the car," said Nasr.

"It was what I was expecting though, very different to any other car I have ever driven..." For world champions Red Bull, who endured a torrid time testing in Jerez, they had another frustrating day at the office, with Sebastian Vettel's new teammate Daniel Riccardo only completing 15 laps.

He came in seventh in the time sheets, a full six seconds off the pace set by Rosberg and Mercedes as Red Bull with their Renault engines continued to struggle with technical issues.

"In all honesty it wasn't a great day today, I guess we still need more time," shrugged the Australian.

"We have four more days coming up and hopefully we can maximise them," added the Torro Rosso recruit.

"At this stage we just need laps..." This was the conclusion of four days testing with the final batch of tests scheduled back in Bahrain next Thursday ahead of the first race of the season, the Australian Grand Prix in Melbourne, on March 16.