The Northern Irishman produced a two-under 69 to take him to one under par and a tie for 50th place in Illinois.
The four-time major winner needs a top four finish at the tournament to reach the Tour Championship, the final event of the campaign.
McIlroy, who runs the risk of missing out on the finale, finds himself 15 shots off the pace with Marc Leishman leading the way on 16 under for the tournament.
Having made a slow start in Illinois, McIlroy is determined to keep battling to try and earn a spot in next week's competition.
"It's hard because I know that realistically there's not much chance of getting in next week, but I still want to go out there and play well," McIlroy said.
"I'm here, so I might as well try to play well and try to get the most out of myself."