BARCELONA (Reuters) - Lewis Hamilton led a Mercedes one-two in Spanish Grand Prix practice on Friday while Red Bull's world champion Sebastian Vettel was sidelined for most of the two sessions by an electrical problem.
Race favourite Hamilton was quickest in both, 0.449 of a second faster than championship-leading team mate Nico Rosberg in the afternoon and 0.868 ahead of McLaren's Jenson Button in the morning.
"I can feel the positive step forward that we've made with the car," the 2008 champion from Britain told reporters. "Our two practice sessions today went very smoothly, in fact I've not had such a good Friday for a very long time."
While Hamilton pounded out the laps, ahead of a weekend that could bring him a fourth successive win, quadruple champion Vettel was little more than a spectator at the Circuit de Catalunya.
The German, winner of nine races in a row at the end of last season, was forced to park up by the side of the track early in the day with just four laps completed.
Marshals handed him a fire extinguisher as he inspected the car before hitching a ride back to the paddock on a scooter.
Red Bull have given the 26-year-old a car with a different chassis to the one he has used so far in what has been a difficult start to the season but there was no obvious improvement.
"We had a simple failure in one of the (wiring) looms this morning which caused a short (circuit) and meant the car stopped. We had to change the chassis loom which is a big job so I wasn't able to go out this afternoon," said Vettel.
"It's a small failure but a big consequence, there was nothing new on that part of the car, it was something new that happened in that area."
The last time the German was at a Spanish circuit, testing at Jerez in January, he spent most of his time in the garage watching mechanics work on the car as Renault wrestled with teething problems on the new V6 power unit.
Hamilton, four points behind Rosberg after four races, went from strength to strength with a fastest lap of one minute 27.023 seconds set six minutes from the end in the morning. After lunch, his best time was 1:25.524.
The impressive margins in the Spanish sunshine underlined his status as the man to watch in Formula One's first European race of the season.
Rosberg, who had said on Thursday he just needed a "normal weekend" to regain the momentum, suffered a cooling issue that cut short his first session and left him fifth fastest before normal service was resumed in the afternoon.
Vettel's Australian team mate Daniel Ricciardo, who has made an impressive start with the team, was third in both sessions with Ferrari's Fernando Alonso, the local favourite for his home race after winning last year, fourth.
"I think we have shown improvement. We want to close the gap to Mercedes but they don't seem to be showing any signs of slowing down," said Ricciardo.
Alonso felt Pirelli had been too conservative with their tyre choices for the weekend and said Ferrari faced a lot of work to get the most out of the car.
With the opening long-haul races in Asia and the Middle East out of the way, all teams have brought updates to their cars and Friday's practice sessions were being closely watched for any signs of change in the pecking order.
It was also the first chance for many of the fans to hear the new, less noisy engines with engineers already looking at ways of making them louder.
In a bizarre early incident, a wing mirror on Sergio Perez's Force India broke off and was left flapping around on a wire. The Mexican completed the lap with it in his hand.
(Editing by Ed Osmond)