Jenson Button, driving for the newly formed Brawn GP team, ended the first of four days of testing fourth fastest after an encouraging day in Barcelona.
Button and Brazilian Rubens Barrichello have retained their places for this season in the new Mercedes-powered car.
But he finished fourth behind the BMW Sauber of Nick Heidfeld, Ferrari's Kimi Raikkonen and Toyota's Jarno Trulli.
"I am so happy the team is on the grid and that's the most important thing for me," said Button.
Button, who has taken a big pay cut to stay in Formula One, spent the morning at the top of the timesheets.
He added: "Whatever people have said about drivers in the past, and me personally, I'm here to race.
"I love racing, this is the best place to be doing it and you've got to make a lot of sacrifices.
"But I'm not the only one, we all do, especially at this difficult time. We all have to make sacrifices."
Owner Ross Brawn said he hopes the team make a "respectable" start to the season in Australia on 29 March.
And the early signs from the Circuit de Catalunya, home of the Spanish Grand Prix, were promising.
Button's fastest lap for Honda in last year's Spanish Grand Prix was 1:22.353 but he recorded an unofficial time of 1:21.140 in the Brawn car, running without branding in its new black, white and yellow colour scheme, on his first full day driving since 19 November.
Honda Racing decided to withdraw from F1 in early December as a result of the global financial crisis, but ex-team principal and former Ferrari technical director Ross Brawn confirmed last Friday that he had taken over the team.
McLaren's partners Mercedes have replaced Honda as the engine provider, with the new V8 unit believed to be providing around 60 horsepower more than the one used by Button last year, and they are hoping for a big step up in performance. BBC Sport Report...
Thats great News
, especially if hes been at the top of the time sheets.
Shows how crap Honda were, if the Mclaren engine has around 60bhp more !!!