Nico Rosberg fastest as Ferrari hit trouble in Bahrain GP practice
Nico Rosberg out-paced Lewis Hamilton as Mercedes dominated practice at the Bahrain Grand Prix.
The German was fastest in both sessions on Friday, ending the qualifying simulations in the evening 0.241secs quicker than the world champion.
Jenson Button was an encouraging third fastest for McLaren-Honda, ahead of Toro Rosso's Max Verstappen.
Ferrari's Kimi Raikkonen and Sebastian Vettel were fifth and sixth, well over a second down on Mercedes.
Ferrari worried about engine reliability?
Vettel was 0.218secs off Raikkonen and 1.6secs behind Rosberg after admitting he "messed up" his best lap with a number of errors.
There was not much evidence that Ferrari might be able to take the fight to Mercedes this weekend, with the Italian cars lagging some way behind the silver ones for the vast majority of the day.
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Vettel did start his race-simulation run on the 'soft' tyres promisingly, lapping at a pace close to Mercedes for the first time all day.
But before a full picture emerged of the two cars' comparative pace the Ferrari ground to a halt out on track with 15 minutes of the session remaining.
Vettel told the team over the radio that the rear left wheel was not properly attached to the car.
After getting out of the car, he added that the team were not yet sure what had happened.
"I felt out of Turn Four I lost drive, so I decided to stop. We don't want to damage or risk the car at that point.
"Mercedes are the benchmark. They are the ones to beat but hopefully we can be a bit closer. The car feels all right and there are bits and bobs we can improve."
Mercedes believe they have evidence that Ferrari are protecting a fragility in their engine by restricting as much as possible the amount of time they run it at full power.
Hamilton no answer for Rosberg
At Mercedes, Rosberg, who won the first race ahead of his team-mate, had an edge over Hamilton all day, with the Briton unable to match his team-mate's pace on one lap. Hamilton did look competitive on his race-simulation run, however.
Hamilton said he was struggling to find balance and lost 0.1secs because of traffic on his quickest lap, so the gap to Rosberg was more like half of what it appeared but that it would be hard to make up that time because it is one of Rosberg's strongest circuits and not one of his.
Despite his apparent struggles, Hamilton said it had been "a good Friday overall", adding: "The car is performing really well here. There were no surprises or issues to report throughout both practice sessions, which is always a good sign.
"There's some work to do tonight in terms of finding a few improvements from the car and also my driving style. I'm looking forward to getting back out there and making some more progress tomorrow."
Shortly after Vettel's car stopped, there was further drama as Romain Grosjean's Haas appeared to suffer a front nose failure.
Button's position in third place is unlikely to be repeated in qualifying on Saturday, but it was further evidence that McLaren and Honda are making significant progress after a difficult 2015.
Button's long-run pace was also relatively encouraging as the team prepared for the race.
The Englishman is being partnered by McLaren's reserve driver Stoffel Vandoorne this weekend after Fernando Alonso was ruled out by FIA doctors as a result of chest injuries sustained in his 180mph barrel-roll accident in Australia two weeks ago.
The Belgian, who flew into Bahrain overnight from Japan and had not driven this year's McLaren car until the start of first practice, ended the session in 11th place, 0.7secs behind Button.
Red Bull and Williams completed the top 10 in alternating positions with Red Bull's Daniil Kvyat seventh fastest, ahead of Williams' Valtteri Bottas, then Daniel Ricciardo and Felipe Massa.
Meanwhile, Renault driver Kevin Magnussen, who was 16th fastest, will have to start the race from the pit lane after being penalised for missing a call to have his car weighed during the second session