Michael Schumacher tried out his 2012 Mercedes for the first time on Tuesday, setting the sixth fastest time in a Formula One testing session, well behind leader Sebastian Vettel's Red Bull.
Vettel, the two-time defending F1 champion, was the quickest at Catalunya Circuit with a lap of 1 minute 23.265 seconds to finish nearly nine-tenths of a second ahead of seven-time champion Schumacher.
Nico Hulkenberg of Force India trailed Vettel by just under two-tenths of a second. Lewis Hamilton of McLaren and Daniel Ricciardo of Toro Rosso were next, with Ferrari's Fernando Alonso edging out Schumacher for fifth.
Lotus, meanwhile, abandoned the session with a chassis problem that will keep it from testing this week, with the track available through Friday.
"Not running this week has been a tough decision to take, but we feel that our choice is the right one," said Lotus team principal Eric Boullier, whose outfit - the team formerly known as Renault - produced the fastest of the 2012 designs in the season's first test at Jerez last week.
Mercedes used its 2011 car at the first test session in Jerez last week, and Schumacher's initial feelings were "good" from his first drive in the F1 W03 Silver Arrow.
"It's certainly an improvement and we have worked hard on the areas we had defined last year," said Schumacher, who is looking for a big improvement in the third season of his comeback. "For my part, I am eager to fight again, looking forward to the new season - and I can't wait for it to begin."
Sergio Perez was seventh despite his Sauber experiencing technical problems, while Vettel complained about his traction grip as cars become accustomed to Pirelli's new batch of tyres due to regulation changes.
"It's clear that the cars have less grip than last year, but we try to come back from that slowly," Vettel said. "It's difficult to judge where we are compared with the others right now. It's all based on feeling, and the feeling I have inside the car at the moment is fine."
The season opens at the Australian Grand Prix in Melbourne on March 18.