João Barbosa and the Action Express Racing squad held a narrow 1.682 second lead over DPi rivals Wayne Taylor Racing as the 12 Hours of Sebring reached its halfway point.
Barbosa was 20 seconds clear of Ricky Taylor in the WTR Cadillac DPi-V.R until a sixth-hour full course caution bunched the field up.
The stoppage – caused by tyre wall contact for Nicky Catsburg's BMW M6 – put Taylor on the bumper of Barbosa's Cadillac DPi.
12 Hours of Sebring: Positions after six hours
The two Cadillacs are the only cars left on the lead lap, following a hard-hitting opening stanza for the Prototype runners.
A notable casualty was the pole-sitting Rebellion Racing ORECA-Gibson, which fell by the wayside through a lengthy alternator change.
The 2016 race-winning ESM crew also encountered trouble, with the #2 Nissan-Onroak DPi suffering from early boost issues.
Other Prototypes that have faltered include the Visit Florida Racing Riley Mk.30, the #22 ESM Nissan-Onroak DPi, and the pair of factory-run Mazdas.
The sister Action Express Racing Cadillac DPi slipped out of the lead battle when Eric Curran clipped the #22 Nissan into a spin at the final corner during hour three. Curran was unable to restart the car and lost a lap as a result, putting the car three laps down after six hours.
Curran's team-mate Dane Cameron is currently running the #31 car fourth overall, with the third-placed JDC-Miller ORECA-Gibson two laps further up the road but one lap off the leaders.
In GTLM, Ford held station at halfway with the #67 car of Scott Dixon still commanding the category, although Stefan Mücke lay just 2.2 seconds behind in the sister #68 car.
Completing the top five at halfway were the Risi Competizione Ferrari 488, the #911 factory Porsche 911, and the #66 Ford GT.
The #3 Corvette had been running consistenly in the top three but a mis-timed pit stop coinciding with the sixth-hour caution pushed Antonio Garcia down to seventh.
Prototype Challenge has been led for some time by Patricio O'Ward in the Performance Tech Motorsports ORECA FLM09, whilst Richard Antinucci paces the GT Daytona class in the Grasser Racing Lamborghini Huracan GT3.
