The Penske-run #7 Porsche 963 of Campbell was the first car to duck below the 1m36s barrier, turning a 1m35.802s after 20 minutes to eclipse the Cadillac V-LMDh cars of Alex Lynn and Alexander Sims by over 0.4s. Ricky Taylor moved up to also shade the Cadillacs, but was still over 0.33s off top spot.
His full-time partner Albuquerque cut the deficit back to 0.172s with half an hour to go, pulling clear of the Ganassi and Action Express Racing Cadillacs.
Colton Herta took the #25 BMW M Hybrid V8 to within 0.66s of the top spot, ahead of the second Porsche, the Meyer Shank Racing Acura and the #24 BMW which turned the fewest laps.
Oliver Jarvis of Era Motorsport and Rinus VeeKay in the #11 TDS Racing car were the early pacesetters, but after the first red flag, Ben Hanley in the Crowdstrike Racing by APR machine jumped to the top with a 1m38.801s effort.
Then Alex Quinn in the PR1/Mathiasen entry took quarter-second off that time although Hanley sliced that down to 0.108s, with Scott McLaughlin’s Tower Motorsport car a further 0.33s behind that, ahead of Jarvis’s early pacesetting effort.
Matthieu Vaxiviere in the #88 AF Corse car moved up to fourth in the closing stages of the session and then moved ahead of McLaughlin by a mere four-hundredths, but found a clear enough lap to bounce into top spot by 0.139s in the dying minutes.
Mikkel Jensen improved the #11 TDS car’s time in the final stages but remained sixth.
Rasmus Lindh in the Andretti Autosport Ligier delivered 1m42.559s with less than 25 minutes to go, 0.325s ahead of Wayne Boyd in the #17 AWA Duqueine, with the other AWA of Matthew Bell third.
The new Ferrari 296 of Cetilar Racing caused the first red flag of the session when it went off at Turn 5 in the hands of Roberto Lacorte.
Halfway through the session, Daniel Juncadella was sitting top of the GTD Pro pile in the WeatherTech Racing Mercedes AMG GT3, and that was the fastest GT3 car overall, until Frederik Schandorff in the Inception Racing McLaren 720S clipped a mere 0.003s off that time.
Parker Thompson in the GTD Lexus RC F was within the same tenth, ahead of Luca Stolz in the SunEnergy1 Racing Mercedes-AMG GT3 and Danny Formal in the Racers Edge Acura NSX.
Mikael Grenier moved to the top of GTD by 0.002s in the Team Korthoff Mercedes, and he had just been beaten by Jules Gounon in the WeatherTech car, before Grenier pulled a quarter-second clear. Maro Engel in the WeatherTech car got the gap down to under 0.2s, and then finally went top with a 1m46.011s, the car having also turned the most number of laps (48) out of all the GT3 machines.
Mario Farnbacher gave Gradient Racing a strong showing with its Acura NSX less than 0.4s behind Grenier, splitting the two Vasser Sullivan Lexus of Parker Thompson and Jack Hawksworth.
As usual Spencer Pumpelly did a fine job to get the Magnus Racing Aston Martin Vantage into fifth in GTD, seventh overall in GTs.