GRANOLLERS, Spain (AP) Lewis Hamilton posted the fastest time in the second practice session Friday at the Spanish Grand Prix, taking over from teammate Nico Rosberg after he was quickest in the morning on another dominant day for Mercedes.
Hamilton, who leads Rosberg by 27 points and Ferrari driver Sebastian Vettel by 28, was .408 seconds faster than Vettel and .764 ahead of Rosberg in the afternoon.
''We've not analyzed the times yet but our pace felt pretty good,'' said Hamilton, the defending champion and early-season leader. ''The priority was understanding the tires and how they wear.''
Rosberg, who has won only one of his past 15 races, had the fastest lap of 1 minute, 26.828 seconds in the morning session - a fraction ahead of Hamilton and nearly one second better than Vettel in warm, sunny and slightly windy conditions.
''I worked a lot on the race pace in the afternoon and was happy with my long run, where we seemed to be quicker than the Ferraris,'' said Rosberg, who complained about the quality of the Catalunya circuit. ''The track was very poor. I don't know really why, but hopefully it will be more predictable for qualifying and the race.''
Ferrari driver Kimi Raikkonen was fourth in both sessions as the season's two dominant teams again proved to be better than the rest.
There is a third practice on Saturday ahead qualifying, where Hamilton will bid for his fifth straight pole position this season.
Considering the Catalunya track is arguably the hardest on which to overtake in F1, Rosberg will likely struggle unless he takes his first pole since the season-ending Abu Dhabi GP in November. His last victory came in the previous race, at the Brazilin GP, where he won from pole.
The problems continued for Red Bull as Daniel Ricciardo only got on track near the end, and the Australian driver has now used his four allocated engines for the season.
But there were encouraging signs for McLaren, which has been beset by power unit failures since switching to Honda engines this season, with Jenson Button seventh quickest in the afternoon.
McLaren has yet to score a point so far this season - with Fernando Alonso's 11th-place finish the best so far - but upgrades to the car and engine could signal an improvement this weekend.
''It's the closest we have been all year,'' said Button, who sat out the Bahrain GP three weeks ago because of an electrical fault. ''There is a lot of work for us to do in every area to optimize what we have. It has been a good day, but lots to do to figure out why we can't do certain things.''
On an otherwise quiet day, there was one dramatic moment when French driver Romain Grosjean's engine cover disintegrated, sending debris spewing out from the back of his Lotus and bringing the session to a temporary halt.
Although Ricciardo struggled with his engine reliability, there was better news for teammate Daniil Kvyat of Russia, who had the fifth fastest time in the afternoon.
In the morning, British driver Susie Wolff - who is a development driver for Williams this season - was 14th.