Fernando Alonso got behind the wheel of a Renault for the first time since re-joining the team from McLaren.
Alonso returned to the Renault cockpit for the first time since 2006
Alonso, who won both his world titles with Renault, tested last season's car in Jerez and was optimistic about the team's chances in 2008.
"The first day of work was very positive," said the 26-year-old.
"There have been some big changes at Renault. They've strengthened many areas, so in one sense it is a new adventure with a new team."
Alonso signed a two-year deal with Renault after a tempestuous season at McLaren which was marked by an intense rivalry with his team-mate Lewis Hamilton.
The Spaniard is anticipating a more harmonious relationship at Renault, who he joined as a test driver in 2003, but is approaching the season determined to succeed.
"There were a lot of good feelings, good memories and plenty of optimism," Alonso said.
Towards the end of my runs I started to gradually take a few more risks with the car
"I don't anticipate problems with anybody in 2008 and (Renault team boss) Flavio Briatore has always demonstrated that he can manage these situations very well.
"At the start of any season, the only thing I want to do is work hard, and to give maximum effort to achieve my potential.
"I know I can do this successfully with Renault, because we have managed it in the past.
"I want to give everything I can, and I know the team will do everything to win; this is always our approach to the sport."
Renault's new R28 car will hit the track for the first time in Valencia on Monday before its official launch in Paris on 31 January.
However, last season's car was adapted for testing by removing the driver aids which have been banned under new F1 rules and Alonso is confident he can handle the changes.
"This was the first time that I have driven without traction control, but towards the end of my runs I started to gradually take a few more risks with the car," he said.
"I am convinced that after two or three races, we will have completely forgotten how it was to drive with driver aids.
"It's about the driver finding his limits and adapting his driving style."
Renault were one of seven teams testing in Jerez, including BMW Sauber and McLaren, who were both trying out their new cars.
And although testing times are largely meaningless, Alonso was given a fillip by setting fastest lap of the day, 0.147 seconds quicker than McLaren test driver Pedro de la Rosa. Ferrari's world champion Kimi Raikkonen was third.
BMW driver Robert Kubica said: "It was a good start. I've only driven a few laps so far, but I have the impression that everything is heading in the right direction.