Bastian Buus in the #22 Allied Racing Porsche set the early pace in the opening practice session at Spa-Francorchamps.
The two Allied Racing Porsches dominated the session and were first and second for the entire session, underlining their intention to end the title battle here in Belgium.
Nicolaj Møller-Madsen was fastest in the first half of the 60-minute practice session, but in the end it was his young compatriot Bastian Buus who put in an even faster lap of 2m33.113, almost four tenths quicker than his teammate.
Alfred Nilsson in the #98 Lestrup Racing Team BMW M4 GT4 completed a Pro-Am and overall top three on the 7.004 km Belgian circuit.
Fourth fastest overall and setting the pace in the Silver class was Duncan Huisman in the #99 V8 Racing Chevrolet Camaro. Huisman was 0.261s faster than fellow Dutchman Willem Meijer in the #30 Selleslagh Racing Team Mercedes-AMG.
Completing the top six was Xavier Lloveras in the #15 NM Racing Team Mercedes-AMG. Lloveras is joined by Rafael Villanueva here at Spa-Francorchamps.
Gael Castelli (#8 CMR Toyota), Thijmen Nabuurs (#17 V8 Racing Chevrolet), Theo Nouet (#27 AGS Events Aston Martin) and Nicolas Gomar (#89 AGS Events Aston Martin) completed the overall top 10, with Gomar setting the fastest lap in the Am class.