Until Monday, Button had not properly driven an F1 car since 19 November
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. Button's time was 0.8secs slower than Heidfeld's best.
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.
In 2008, Honda had a miserable season with Button scoring just three points from 18 races.
But after Honda announced their departure Brawn said the new car was considered a potential race winner.
And 29-year-old Button admitted that racing for the newly formed team had given him a huge lift and a different perspective on his career.
"I wouldn't say in the last few years it all went a bit stale but, when something happens like that (Honda pulling out), it's like 'Wow!' and it does become a fresh start," said Button.
We all do feel like kids again
"Getting into a Formula One car now, I feel like a kid again. It's so exciting.
"It feels like 2000 again (the year he started out in F1) but with nine years of experience, so it's the perfect position to be in.
"I have the experience but I have the hunger to achieve and I'm not the only person that feels like that within the team.
"It's all of us, especially after having produced a car I think is competitive and very different to the last two cars.
"We all do feel like kids again and you can see that within the team, the smiles on people's faces, and that was before we drove the car today.
"There's a great atmosphere and it's brought everyone that much closer together, and it's a great thing to be a part of."
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.