Jerez: Mercedes' 2011 car topped Formula One pre-season testing for a second straight day, while Lotus were the quickest of this season's new batch of competitors on Thursday.
Nico Rosberg was quickest around the Jerez circuit with a lap time of 1 minute, 17.613 seconds to follow up Michael Schumacher's leading pace a day earlier.
Romain Grosjean in a Lotus was the fastest in the 2012 designs, the French driver's time of 1:18.419 more than eight-tenths of a second faster than two-time defending world champion Sebastian Vettel of Red Bull, who was driving for the first time.
Lewis Hamilton's first spin in the new McLaren placed him fourth ahead of Jean-Eric Vergne of Toro Rosso, while Fernando Alonso kicked off his Ferrari campaign with the seventh-fastest time.
Grosjean's time was the fastest of the 2012 cars so far after three days of testing, with teammate Kimi Raikkonen having topped the timesheets on Tuesday's opening session.
"I am one of the happiest guys in the world right now," said Grosjean, who returns to the former Renault team after competing in GP2 for one season. "For sure there's margin for improvement in the tests ahead, but it's a great start to the year."
Cool morning temperatures delayed optimal running for most as cars needed time to warm up tyres that all teams are still getting accustomed to. Force India reserve driver Jules Bianchi skidded off the slippery circuit to keep Nico Hulkenberg from getting any laps in the afternoon, when he was meant to hand over.
"The car feels fine and as we expected," said Vettel, who was back on track after dominating the championship last season. "There's quite a loss of grip due to the regulation changes, but you get used to it quickly and then start to explore how the new car feels. So far we can be happy, we had a couple of issues, but it's normal when the car is new."
Hamilton took over McLaren duties from Jenson Button, and the 2008 champion had few complaints on a day when he managed 80 laps.
"My initial feelings are pretty positive. Despite going through a long, tough programme of initial tests there were no nasty surprises, which is pleasing," Hamilton said. "The car feels reasonably quick, and that's a nice feeling."
Ferrari's pre-season form has failed to impress even with Alonso replacing Felipe Massa in the driver's seat on Thursday, with the Italian team off the pace going into Friday's fourth and final day of the test in southern Spain.
The season begins with the Australian Grand Prix on March 18.