Pla lapped the 3.56-mile course in 1min33.324sec, while Ricky Taylor and Juan Pablo Montoya were only 0.219 and 0.241sec behind.
Ex-Formula 1 driver Felipe Nasr also moved Action Express Racing’s Cadillac DPi-V.R ahead of the second Mazda of Jonathan Bomarito to claim fourth, 0.087sec behind Montoya.
The two JDC-Miller Motorsports Cadillacs of Tristan Vautier and Joao Barbosa were next up, as the team appears to have taken a step forward in 2020, while Renger van der Zande of Wayne Taylor Racing was eighth, but still within a second of top spot.
James Calado’s Ferrari 488 GTE set fastest time in GT Le Mans by just 0.001sec ahead of the Porsche 911 RSR of Nick Tandy, lapping in 1min42.685sec on his fourth lap.
Corvette Racing revealed the promise of its all-new mid-engined C8.Rs as Tommy Milner was only 0.108sec in arrears, just ahead of the two BMW M8s of Jesse Krohn and Philipp Eng which were split by the second 911 of Laurens Vanthoor.
The entire GTLM field was covered by 0.45sec.
Seventh and final hour-long practice begins at 2.45pm local (Eastern) time, and is for all classes.