Audi led the way in free practice ahead of Saturday's second round of the World Endurance Championship at Spa.
Andre Lotterer posted the fastest time of the day aboard the No. 1 Audi R18 e-tron quattro with a 2m01.276s lap late on in the second of the pair of two-hour sessions. That time, set in the high-downforce R18, eclipsed by two tenths the previous fastest time set by Lucas di Grassi in the new long-tail version of the car Audi is planning to run at the Le Mans 24 Hours next month.
Audi Sport Team Joest technical director Ralf Juttner said: "Andre didn't set the time on new tires, though he didn't have a lot of fuel for sure. But it is close: don't forget that this is a seven-kilometer [four-mile] track. The longtail isn't far away as our simulations suggested, but it is not full low-downforce and the other cars are not running as much downforce as they were at Silverstone."
Audis blocked out the top three places on the times in each of the two sessions, with the No. 2 car third on a 2m01.690s lap in the hands of Tom Kristensen.
The rest were led by the best of the Rebellion Racing Lola-Toyota B12/60s on a 2m02.305s lap set by Nick Heidfeld. That was just enough to push the fastest of the Toyota TS030 HYBRIDs into fifth place.
Alex Wurz set the best Toyota mark in the 2013-specification TS030, which was just under two tenths quicker than Stephane Sarrazin in the 2012 version of the car.
LMP2 honors were taken by the Jota Sport's Zytek-Nissan Z11SN in the hands of Oliver Turvey. His 2m09.025s lap was three tenths ahead of Olivier Pla in the fastest of OAK Racing's Morgan-Nissan LMP2s.
Ferrari was fastest in GTE Pro courtesy of a 2m20l.159s lap from AF Corse driver Gianmaria Bruni, which was two tenths up on the best Aston Martin driven by Stefan Mucke.
Matt Griffin topped the GTE Pro classification in his AF Corse Ferrari.