Saturday started with an alarm call for the Mercedes mechanics, when the power unit in Nico Rosberg's W05 sent in a sick note first thing and decided it wasn't happy coming out to play today.

"It was difficult because we had to change engines this morning," Rosberg told reporters at the Bahrain International Circuit. "We noticed about five o'clock [in the morning].

"We have two different groups of mechanics, one group works through the night and the other works in daytime, and the night group had just finished the car, building it together, and then at five o'clock they just switched the engine on to see that it worked and it made a strange sound so they realised that they had to change it. That took some time."

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Finally the engine change was complete and Rosberg was able to head out at around 11.30am to catch the final couple of hours of the morning session, and the whole of the afternoon session.

"From then it was a great day really, lots of laps, learned a lot of things, so it was really good," he enthused, after going on to finish in second place on the day's timesheets behind an impressive turn from Felipe Massa in his new Williams.

"The only problem was the conditions out there today: very windy, sandstorm - first time in Bahrain I've experienced that on the track, so that was quite crazy. You get to the next corner and all of a sudden there's a sandstorm there and you've got no grip! That was tough, but otherwise okay."

Although he spent much of the afternoon contesting the top spot with Massa, Rosberg said that finishing with the best time on day three hadn't been important to either him or the team.

"Testing is about getting the best practice for Melbourne and it's always good to practice with lower fuel because qualifying is an important part of the weekend," he explained. However, he was clearly impressed with how fast Massa had proved to be on Saturday.

"The Williams is looking decent, definitely," added Rosberg. "I think a lot of it is probably down to the powertrain. It seems that the Mercedes powertrain has a little bit of an advantage over some others at the moment, possibly - I think that could be one of the [factors]."

All of which makes if difficult for everyone - drivers, team personnel, media and fans alike - to get any sense of the relative standings of the different teams with just one day of pre-season testing remaining.

"I'm not worried about anybody at the moment, just concentrating on our own stuff and just getting the most out of it," Rosberg said. "The last day of testing and then we'll see how we are in Melbourne, but I'm optimistic for a good start to the season."

With Sebastian Vettel once again unable to turn a lap during the day in the problem-hit Red Bull, and Lotus also wrapping the day early after hitting problems, it seemed that despite their early engine glitch it was Mercedes who were continuing to have the best form of the big teams. However, Rosberg was quick to quell speculation that the Brackley-based squad were shaping up as the team to beat heading into the 2014 world championship.

"We're optimistic, but to say exactly where we are [compared with others] would not make sense," he said. "I don't want to guess because everyone is on different fuel levels and as we know it's important to be careful with testing results. I hope we can have a good start to the season - it's possible, but reliability is an issue for everyone, us too. It's going to be tough.

"There's no point to having an impression," added Rosberg. "It depends so much on fuel levels and everything so I don't want to have an impression. At the time I was a lot quicker, so [the] overall picture is that I'm very pleased ... My car is not 100 per cent ready yet but we still have a few days.

"I don't want to see us as the top team," he emphasised. "I don't want to say that because it's too early to say that. Testing is testing, I'm optimistic for the beginning of the season, yes - we've had some good tests - but reliability is still a concern for us as it is for everyone else. It's tough to get it 100 per cent."

Rosberg did however agree that he was fully expecting to be competing for the first Grand Prix win of the season when F1 arrives in Australia next month. "I'm always disappointed if we're not competitive," he said. "I think we might be in a better position than some of the other teams but still you have to get to the end of the race and that's a big challenge."

"It's a great country, a lot of fans there, so it's nice that we're going to race there," he added.