Britain's Jenson Button put the new Brawn GP Formula One team on top of the testing timesheets in Barcelona on Wednesday while champion compatriot Lewis Hamilton had a difficult day with McLaren.
Button, whose car has been both quick and reliable since the former Honda team's test debut on Monday, completed 130 laps with a best time of one minute 19.127 seconds according to unofficial timings.
That was more than a second faster than the next quickest driver, Ferrari's Brazilian Felipe Massa, at the Circuit de Catalunya.
Button, who completed a full race distance with live pit stops in the morning before carrying out a qualifying simulation in the afternoon, had also lapped fastest in Monday's morning session.
"It's been another great day and a very positive test generally," the Briton, whose team only secured their future last week after a management buyout led by principal and new owner Ross Brawn, said in a team news release.
Honda announced in December that they were pulling out due to the credit crunch.
"It's so good to have achieved a full day's running, including a race distance, without any issues and it's fantastic that we clocked up so much mileage," added Button.
"After the challenges we have faced, to come here and get off to such an encouraging start is just what we all needed and I can't thank the team enough."
While Button smiled, Hamilton ended the day at the bottom of the timings after 86 laps.
The 24-year-old, whose team have appeared lacking in pace in recent days, went off at turn two in mid-afternoon and hit a tyre barrier. The impact broke off the front wing end plate and damaged nose struts.
BMW-Sauber's Polish driver Robert Kubica was third fastest with Toyota's German Timo Glock fourth and Spain's double world champion Fernando Alonso fifth for Renault.