The Italian set the benchmark time inside the first third of the 50-minute session, a 2m04.781s putting him four-tenths clear of early pacesetter Maro Engel (#77 Craft-Bamboo Mercedes).
Kelvin van der Linde (#222 Valvoline Audi) closed the gap to 0.01s moments later, that proving to be as close as anyone could get to the leading Mercedes.
The #999's cause was helped by an interrupted end to the session, with a 10-minute red flag stoppage to clear a pair of cars stopped on track.
One was the #12 EBM Porsche, believed to be a mechanic issue, while the other was the #77 Mercedes which appeared to have made contact with the wall.
By the time the session restart there was just five minutes to go.
Despite the suspected trip into the wall Engel still ended up with the third fastest time, a 2m05.144s, followed by the #22 Valvoline Audi.
The #1 EBM Porsche was the best placed 911 GT3 R in fifth, the #2 Valvoline Audi capped off a solid first hit-out for the R8s in sixth, while the Triple Eight Mercedes was seventh.
The #7 M-Sport Bentley was eighth, the #63 FFF Lamborghini ninth and the #18 KCMG Nissan 10th.
The track action kicked off with an additional half-hour session excluding Platinum and Gold rated drivers, which proved to be a rollercoaster for the #35 KCMG Nissan.
Josh Burdon both topped the times and caused the session to be cut short, the Aussie hitting the wall at Reid Park shortly after going quickest with a 2:06.553s.
The car didn't make it out in the second session.