Mercedes driver Nico Rosberg posted the fastest time of the first Bahrain pre-season test as rivals Red Bull endured more mechanical issues, which reduced the constructor's champions to only a handful of laps on track.
The German racer registered a lap time of 1:33.283, 1.6 seconds clear of his nearest challenger Jenson Button in the McLaren.
Red Bull's Daniel Ricciardo completed only 15 laps as the Milton Keynes-based outfit spent much of the final day behind screens, with mechanics trying to solve the car's reliability issues.
Such is the speculation surrounding the performance of their RB10 that Sky Sports News quotes a senior figure from a rival team as suggesting that Red Bull may introduce a 'b-spec' version by the time of the Spanish Grand Prix in May.
The grid have a second four-day test in Bahrain to fine-tune their packages before the season-opening Australian Grand Prix on March 16.
