By Alan Baldwin
BARCELONA (Reuters) - Formula One world championship leader Nico Rosberg set the practice pace on Friday for a Spanish Grand Prix that could see the Mercedes driver celebrate his eighth win in a row.
The German, who would become only the third driver to win the first five races of a season after compatriot Michael Schumacher and Britain's Nigel Mansell if he triumphs on Sunday, led the way with a fastest time of one minute 23.922 seconds.
Ferrari's Sebastian Vettel had been fastest in the morning session in 1:23.951 and was fourth in the second session.
Both were well inside Rosberg's 2015 pole position time of 1:24.681.
Vettel's team mate Kimi Raikkonen was second fastest in both sessions, 0.254 adrift of Rosberg in the afternoon, with three-times world champion Lewis Hamilton fourth in the morning but third after lunch.
Ferrari were the only ones using the quicker, soft tyres in the morning, having opted for more sets than their rivals as they feel their car is better suited to them.
Hamilton is 43 points behind Rosberg in the standings after suffering power unit failures in the last two race weekends that sent him to the back of the field in China and to 10th on the starting grid in Russia.
Mercedes said they had changed the turbo and a part of the energy recovery system for Barcelona.
But limited time to change the design since the last race in Russia meant they had just one example available and getting it ready in time had taken a "superhuman" effort, the team said.
Dutch teenager Max Verstappen was sixth and eighth on his practice debut with Red Bull after leaving Toro Rosso in a swap with Russian Daniil Kvyat. Australian team mate Daniel Ricciardo was quicker in both.
Toro Rosso's Carlos Sainz was ahead of both Red Bulls in the afternoon.
Spain's Fernando Alonso, twice the world champion, was 10th and seventh on another encouraging day for McLaren after last year's struggles.
Mexican Sergio Perez had a difficult morning, stopping with two-thirds of the session remaining after smoke came from the rear of his Force India car. The team have brought major aerodynamic upgrades to Spain.
French rising star and reserve Sebastien Ocon made his practice debut with Renault, taking the car that Britain's Jolyon Palmer will race on Sunday, but was unable to set a lap time due to technical problems.
Palmer also had problems in the afternoon, with a tyre blow out halting the session.
(Editing by Catherine Evans)