Spieth, who will continue his bid for an unprecedented calendar grand slam at the Open Championship in a week’s time, carded a level-par 71.
The 21-year-old American, who has enjoyed major wins at Augusta National and Chambers Bay already this year, shot three birdies and three bogeys to sit eight shots off the lead, held by Justin Thomas and Nicholas Thompson.
Spieth birdied the fifth, but consecutive bogeys on the eighth and ninth and another dropped shot at the 11th saw him fall down the field.
He picked up shots on the 13th and 14th, but was unable to claw back any more of the lost ground.
Spieth’s decision to compete in Illinois, rather than the Aberdeen Asset Management Scottish Open, ahead of the showpiece at St Andrews has made headlines as it is unlikely to prove ideal preparation for the Open.
At the top of the leaderboard, Thomas and Thompson both shot 63s.
Thomas carded 10 birdies, including four in his last five holes, and two bogeys, while Thompson had an eagle at the par-four 14th, seven birdies and a bogey.
Charles Howell III and Luke Guthrie were a shot off the pace on seven under, ahead of a group of five players on six under.