Jesse Krohn and Bruno Spengler wound up just 0.069sec apart when they completed their best laps of 1min15.482 and 1min15.551 in the handsome Rahal Letterman Lanigan-run machines.
Nick Tandy also made a late improvement to place the #911 Porsche in third, 0.124sec behind, as the works 911 RSRs made their return to the series having skipped Mid-Ohio.
Tommy Milner and Antonio Garcia held 1-2 in the Corvette C8.Rs for most of the session but ultimately ended up 0.2sec off the ultimate pace.
Mario Farnbacher put Meyer Shank Racing at the top of GT Daytona class, lapping 2.2sec off the best GT Le Mans pace and a mere 0.046sec faster than Jack Hawksworth in the #14 Lexus RC F of AIM Vasser-Sullivan.
This pair were almost half a second clear of Alvaro Parente in the second Acura, with Paul Holton fourth in the Compass Racing McLaren 720S.
Just before the halfway point of the one-hour session, the drivers lost 13mins of the session to a red flag caused by a crash for the #12 Lexus RC F of Michael De Quesadal, standing in for Frankie Montecalvo.
When action resumed, IMSA officials smartly added 10mins to the original allotted time. Remarkably, Townsend Bell’s best efforts in the #12 Lexus were good enough for him to retain fifth ahead of Pat Long’s Wright Motorsports Porsche 911 GT3R, Lawson Aschenbach in the Riley Motorsports Mercedes AMG GT3 and Bill Auberlen steering the Turner Motorsports BMW M6.
Second practice will start at 9.45am local (Eastern) time.