* Rosberg fastest in second session * Ferraris one-two in morning practice * Toro Rosso's Sainz beats both Red Bulls
Formula One championship leader Nico Rosberg set the pace in Friday practice for a Spanish Grand Prix that could see the Mercedes driver celebrate his eighth win in a row.
While the German had no problems to report, his three-times world champion team mate and closest rival Lewis Hamilton complained about his car's handling after ending third-fastest.
Rosberg, who could become only the third driver to win the first five races of a season on Sunday, after compatriot Michael Schumacher and Britain's Nigel Mansell, led the way with a time of one minute 23.922 seconds.
"I feel very comfortable with the car, with some new parts added and they seemed to have worked well," said Rosberg.
Ferrari's Sebastian Vettel was fastest in the morning session in 1:23.951 and fourth in the second. Both Germans were well inside Rosberg's 2015 pole position time of 1:24.681.
Vettel's team mate Kimi Raikkonen was second fastest, 0.254 adrift of Rosberg in the afternoon, with Hamilton fourth in the morning.
"That wasn't an easy day for me, I struggled with the car balance through the day and this afternoon in particular it felt really tricky to drive," said Hamilton. "The lap times don't tell the full story, but it's clear we have plenty of work to do this evening to get the car driving like I want it to."
Ferrari had been alone in using the quicker, soft tyres in the morning, having opted for more sets than their rivals.
Hamilton is 43 points behind Rosberg in the standings after 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 its junior team Toro Rosso in a swap with Russian Daniil Kvyat. Australian team mate Daniel Ricciardo was quicker in both.
"We got through a lot of work and did a lot of laps and I think that's the most important thing for me at the moment," said Verstappen. "I'm still getting used to the car, I don't feel its full limit yet. It's a new car, but I wasn't nervous this morning."
Toro Rosso's Carlos Sainz was ahead of both Red Bulls in the afternoon.
Spain's Fernando Alonso 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. The team have brought aerodynamic upgrades to Spain.
French 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 had a tyre blowout in the afternoon.
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 "Nico Rosberg Spanish Grand Prix")
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