IndyCar championship leader Simon Pagenaud set the pace for Team Penske in opening practice for Sunday's race at Road America.
The French driver, who has an 80-point lead over Scott Dixon in the standings, set the early pace before improving his time to a 1m43.480s in the closing stages of the 75-minute session.
IndyCar racing returns to Road America this weekend after an eight-year absence for top-level American single-seater racing at the venue.
Andretti Autosport's Carlos Munoz and AJ Foyt driver Jack Hawksworth made late improvements during first practice to move up to second and third.
Scott Dixon finished under half a second down on Pagenaud's quickest time, with Chip Ganassi Racing team-mate Tony Kanaan completing the top five.
Marco Andretti and Indianapolis 500 winner Alexander Rossi were sixth and seventh, ahead of James Hinchcliffe.
Graham Rahal finished ninth despite an early electrical issue with his Rahal Letterman Lanigan Dallara-Honda.
Josef Newgarden returned to action just two weeks after being injured in a high-speed crash at Texas Motor Speedway, when he suffered fractures to his right clavicle and right wrist in a heavy crash with Conor Daly.
Newgarden was given permission to take part in practice but will undergo further tests afterwards.
He finished in 10th place and completed the most laps during the session, with 15.
Pagenaud's Penske team-mates Juan Pablo Montoya, Will Power and Helio Castroneves found themselves down in 11th, 12th and 16th.
Montoya was one of several drivers to spin during the session at Turn 12.
The last Champ Car race in 2007 was won by current KSVH Racing driver Sebastien Bourdais, who finished down in 15th place in first practice.
The first session took place under sunny Wisconsin skies, with drivers reporting high tyre wear.
PRACTICE ONE TIMES
|1||Simon Pagenaud||Team Penske||Dallara/Chevrolet||1m43.4804s||-||11|
|2||Carlos Munoz||Andretti Autosport||Dallara/Honda||1m43.5560s||0.0756s||12|
|3||Jack Hawksworth||AJ Foyt Enterprises||Dallara/Honda||1m43.6747s||0.1943s||15|
|4||Scott Dixon||Chip Ganassi Racing||Dallara/Chevrolet||1m43.8849s||0.4045s||15|
|5||Tony Kanaan||Chip Ganassi Racing||Dallara/Chevrolet||1m44.1246s||0.6442s||9|
|6||Marco Andretti||Andretti Autosport||Dallara/Honda||1m44.1296s||0.6492s||14|
|7||Alexander Rossi||Andretti Herta||Dallara/Honda||1m44.2805s||0.8001s||14|
|8||James Hinchcliffe||Schmidt Peterson Motorsports||Dallara/Honda||1m44.2812s||0.8008s||14|
|9||Graham Rahal||Rahal Letterman Lanigan||Dallara/Honda||1m44.3037s||0.8233s||9|
|10||Josef Newgarden||Ed Carpenter Racing||Dallara/Chevrolet||1m44.4318s||0.9514s||15|
|11||Juan Pablo Montoya||Team Penske||Dallara/Chevrolet||1m44.5035s||1.0231s||13|
|12||Will Power||Team Penske||Dallara/Chevrolet||1m44.5234s||1.0430s||16|
|13||Ryan Hunter-Reay||Andretti Autosport||Dallara/Honda||1m44.5473s||1.0669s||9|
|14||Spencer Pigot||Ed Carpenter Racing||Dallara/Chevrolet||1m44.6766s||1.1962s||13|
|15||Sebastien Bourdais||KVSH Racing||Dallara/Chevrolet||1m44.6940s||1.2136s||14|
|16||Helio Castroneves||Team Penske||Dallara/Chevrolet||1m44.8184s||1.3380s||9|
|17||Max Chilton||Chip Ganassi Racing||Dallara/Chevrolet||1m44.9057s||1.4253s||10|
|18||Takuma Sato||AJ Foyt Enterprises||Dallara/Honda||1m44.9381s||1.4577s||16|
|19||Mikhail Aleshin||Schmidt Peterson Motorsports||Dallara/Honda||1m45.0189s||1.5385s||17|
|20||Charlie Kimball||Chip Ganassi Racing||Dallara/Chevrolet||1m45.3892s||1.9088s||14|
|21||Gabby Chaves||Dale Coyne Racing||Dallara/Honda||1m45.7856s||2.3052s||16|
|22||Conor Daly||Dale Coyne Racing||Dallara/Honda||1m46.0191s||2.5387s||14|