The race for the overall win took a fresh twist in the 19th hour when the leading #51 Ferrari 499P failed to get away immediately after its pitstop as Antonio Giovinazzi handed over to Alessandro Pier Guidi, requiring a power cycle to get going again.
That eradicated an advantage of a minute that Ferrari had over the sole surviving Toyota GR010 Hybrid, the #8 car, which had been bolstered by slow zones that hurt the car driven by Sebastien Buemi.
After that, Buemi closed right in on Pier Guidi, even getting within a second of the Ferrari man at one stage, before the Italian opened the advantage up again.
Buemi handed over to Brendon Hartley at the next round of stops, costing the Toyota ground as Pier Guidi stayed on board the Ferrari, with the two cars then circulating around 20 seconds apart.
However, Toyota mixed up its strategy by bringing in Hartley earlier than the pit schedule that he and Pier Guidi had been running on, sending out the Kiwi with fuel only.
Ferrari reacted by bringing in Pier Guidi the following lap, with James Calado taking over at the wheel and coming under pressure from Hartley as he headed back out on circuit.
The gap between the two leaders shrunk to as little as 1.4 seconds with four hours to go before Calado started to stretch his legs at the head of the field again, and the 21-hour mark, Calado was 14 seconds ahead of Hartley.
The third-placed #2 Cadillac V-Series.R, the only one of the American marque's cars to have enjoyed a clean run up to now, dropped off the lead lap in the 20th hour.
Alex Lynn was running around three minutes ahead of the sister #3 car of Renger van der Zande, which has dropped two laps off the lead.
The #93 Peugeot 9X8 was fifth, four laps down, after taking a nose change as its last pitstop, while the best of the Porsche 963s, the #5 machine, was running sixth ahead of the delayed #50 Ferrari.
Peugeot's #94 machine had just been wheeled into the garage as the clock struck 1pm local time, while the #6 Porsche suffered further lengthy visits to the pits to replace the hybrid battery.
Inter Europol Competition continued to lead the LMP2 class, but with a much reduced gap as the Polish squad was hit with a drive-through penalty for overtaking under the safety car.
Albert Costa had the distinctive yellow-and-green ORECA around 18 seconds ahead of the #41 WRT machine of Robert Kubica.
Third in class was the IDEC Sport machine with pole-winner Paul-Loup Chatin at the wheel, followed by Neel Jani in the Duqueine car.
In GTE Am, Corvette Racing completed its recovery from its early woes to take the class lead in the hands of silver-rated driver Nico Varrone.
The solo C8.R in the field was running some 20 seconds clear of its nearest challenger, the Iron Dames Porsche of Sarah Bovy, with the ORT by TF Aston Martin and the 'Rexy'-liveried Project 1 Porsche still in the hunt.
There were two more retirements in class as both Michael Fassbender (Proton Porsche) and Daniel Serra (Kessel Ferrari) suffered race-ending crashes at the Porsche Curves and Indianapolis respectively.
The Garage 56 NASCAR Chevrolet Camaro ZL1 dropped to the bottom of the leaderboard as it spent time in the pits having its gearbox replaced.
