Mercedes appear to have the edge

Mercedes' power again held sway on the opening day of the final four-day pre-season Formula One test in Bahrain.

With reliability over speed set to be the key component for scoring points during the early part of the season as the teams continue to learn about the new power units, Mercedes appear to have the edge.

Force India, Williams and McLaren - all sporting the Mercedes power unit - each clocked up over 100 laps of the Sakhir circuit.

Force India's Sergio Perez, with 105 laps to his name, posted the quickest time of the day of one minute 35.290 seconds, two seconds adrift of Nico Rosberg's best in his Mercedes from last week's second test.

Valtteri Bottas finished 0.894secs down on Perez, but racked up a day-high 128 laps for Williams as they and Force India concentrated on race runs rather than outright pace.

It was a similar story for the works Mercedes team and McLaren, with Rosberg completing 89 laps for the Brackley-based squad, while Kevin Magnussen chalked up 109 for the Woking-based marque.

Rosberg, however, conceded "it was not a fantastic day" due to "a few small problems".

The German added: "This shows we are pushing everything to the limit and we still have a massive challenge in front of us during the last three days of testing."

Overall, Rosberg and Magnussen were fourth and sixth quickest respectively, with Ferrari's Kimi Raikkonen third on the timesheet and Adrian Sutil in a Sauber - powered by the Prancing Horse - fifth.

Raikkonen, though, only managed 54 laps due to technical glitches, one of which saw him stop on the backstraight in the closing moments, while Sutil completed 89 laps.

As for the troubled Renault-powered cars, there were yet more woes, including for four-times champions Red Bull.

Daniel Ricciardo, with just 53 laps behind him in the RB10 over his previous four days in the car, started off strongly and was up to 39 in a reasonably short period of time.

Exhaust problems again, though, curtailed his running, to leave the Australian seventh on the timesheet, 2.6secs behind Perez.

Regardless, a much happier Ricciardo said: "Behind the wheel that's the best I've felt so far.

"The package seems to be coming together better. We still had a few issues, but at least when the car was on track it was much more useful running and we made progress in the morning.

"It's getting there. I'd love to keep going, but at least Seb (Vettel) has two days after me in the car, so I think we'll be looking pretty good for the next three days."

Toro Rosso's Daniil Kvyat managed 56 laps before a technical failure kept him in the garage in the afternoon.

Lotus tried a new exhaust, but due to overheating problems Pastor Maldonado managed only 31 laps, while Caterham's Kamui Kobayashi ran for just 19 laps.

Marussia, meanwhile, enjoyed their best day on track so far with Max Chilton completing 44 laps, and with a time 3.3secs behind Perez for eighth quickest.

A delighted Chilton said: "It's great to finally achieve some reasonable mileage and also to have some consistency to our running again.

"We have also had some positive signs in terms of reliability, so all in all I'm quite positive. We have a good basis now for the rest of the week."