The 30-year-old Frenchman only confirmed he would be at the track testing for Forsythe Pettit Racing 24 hours earlier, but showed no signs of nerves as he set the pace in 50.788 seconds.
The ex-Super Aguri Formula One racer was just 0.068 seconds quicker than Oriol Servia, who was back with PKV at the track, having inked his race deal for the season with them.
Britain's Justin Wilson was third fastest as he acclimatised well to Newman/Haas/Lanigan Racing less than a week after joining them.
Wilson takes Bourdais’ seat
30/01/2008 AT 16:51
"It was a pretty good day," said Wilson, who has finished as series runner-up to Sebastien Bourdais for the past two seasons.
"I was just learning to work with the team and learning the car. I think everything is going as planned, we will see how tomorrow goes but I'm pleased with what happened today."
Franck Perera, a front-runner in last year's ChampCar Atlantic Series, was fourth quickest for Conquest Racing while Alex Figge was next up.
The American Pacific Coast driver highlighted his improving form by going fastest in the morning, when all the drivers ran race set-ups on their cars, but dropped down the order as qualifying simulation runs were tried out during the afternoon.
Wilson's team-mate Graham Rahal was sixth quickest - and the only man not to put new tyres on during the afternoon- while Alex Tagliani, running with PKV for the first time, was next up.
"I'm ecstatic right now," he said. "I hadn't been in a car since November and in the morning session we were already third. Everything is going so well.
"This team is so professional, we're working on different set-ups for the car and they are very happy with the feedback I'm giving them.
"It's so much easier to work with a team when the chemistry is there right from the start and you don't have to build that up first. I think this team will definitely be contending for the Championship."
Bruno Junqueira was eighth fastest while Mario Moraes - 14th in last year's British Formula Three Championship, was ninth. Both were driving for Dale Coyne.
2003 champion Paul Tracy completed the top ten for Forsythe Pettit while further down the order David Martinez was in action for Pacific Coast.
GP2 refugee Ernesto Viso also had a try-out for Rocketsports, while former Team Australia driver Simon Pagenaud was in Conquest overalls.
Season 2008
Power and Servia in IRL move
13/03/2008 AT 03:00
Season 2008
Forsythe to close doors
29/02/2008 AT 19:17