Jani was the man behind the wheel as the Lola B12 car set the best time of one minute 9.089 seconds.
Lucas Luhr was second fastest in the Muscle Milk Pickett Racing HPD ARX-03a, the German driver taking the Honda-powered car round in 1:09.428 to allow him and co-drivers Klaus Graf and Romain Dumas to start second.
Guy Smith in the Lola-powered Dyson Racing car set the third quickest time, 1:10.176, to put himself, Chris Dyson and Steven Kane third on the grid ahead of their team-mates in the only other category P1 car, Tony Burgess, Mark Patterson and Chris McMurry.
The 10-hour race begins at 11:30am local time on Saturday.
Qualifying results (pos/team/drivers/car/time/fastest driver):
1. Belicchi/Jani/Prost Lola B12/60 01:09.1 Neel Jani
2. Luhr/Graf/Dumas HPD ARX-03a 01:09.4 Lucas Luhr
3. Dyson/Smith/Kane Lola B12/60 01:10.2 Guy Smith
4. Burgess/Patterson/McMurry Lola B11/66 01:11.8 Mark Patterson