Buemi jumped to the head of the timesheets after a 12-minute red flag midway through session aboard the #8 Toyota GR010 HYBRID Le Mans Hypercar with a 2m02.982s that gave him a margin of more than a second and a half over Ferrari driver Antonio Fuoco.
Fuoco also set his time in the #50 Ferrari 499P LMH, a 2m04.632s, straight after the red to sneak ahead of the second Toyota of Jose Maria Lopez.
Lopez had been quickest up to the stoppage, caused by Luis Perez Companc going off at Les Combes in the Richard Mille-sponsored AF Corse Ferrari 488 GTE Evo.
The Toyota driver’s 2m04.642s set at the start of the session left him just one thousandth behind the Ferrari.
Buemi’s time represented an improvement of nearly four seconds for Toyota over the corresponding session at Spa this time last year.
The 90-minute session was red flagged for a second time and did not restart after an incident involving the Vanwall LMH and D’Station GTE Am Aston.
Ferrari took fourth position courtesy of a 2m04.716s from Alessandro Pier Guidi in the #51 entry run by the AF Corse factory team.
Laurens Vanthoor was only a tenth behind as he took fifth position for Porsche in the best of the Penske-run 963 LMDhs with a 2m04.870s.
Cadillac ended up sixth with the best of the Ganassi-run V-Series.R LMDhs in which Alex Lynn posted a 2m05.347s.
Michael Christensen took seventh in the second Porsche on 2m05.807s, while Gustavo Menezes ended up eighth in the best of the Peugeot 9X8 LMHs with a 2m06.259s.
Source: Autosport