Teams were greeted with wet conditions to kick off the critical day of testing at Eastern Creek.
That meant running was somewhat limited for the first 90 minutes as Shane van Gisbergen led the way in the mid-1m42s.
As the morning session approached the two-hour mark times dipped into the 1m41s for the first time courtesy of Brad Jones Racing's Heimgartner.
Times then quickly dipped into the 1m40s as the likes of Will Davison, Anton De Pasquale and Bryce Fullwood as the crossover point to slicks neared.
An hour out from the lunch break drivers began to venture out on slicks, the BJR cars continuing to show pace as times rapidly improved.
Heimgartner was able to improve more than most, a late 1m29.826s helping him to a healthy advantage at the break.
That time is a little over two seconds slower than SMP's unofficial lap record set by Scott McLaughlin in 2019.
Heimgartner's effort put him 1.3s clear of the next best, Erebus driver Will Brown, while Jack Smith capped off the promising morning for BJR with the third fastest time, just two-hundredths behind Brown.
Will Davison was the best-placed Mustang driver in fourth, 1.5s off the pace, followed by David Reynolds – who split his running with Grove Racing endurance driver Garth Tander – and Team 18 pair Mark Winterbottom and Scott Pye.
Matt Payne and Macauley Jones were eighth and ninth with Brodie Kostecki rounding out the top 10.
Triple Eight had a quiet morning in terms of outright pace, the #88 entry being shared by Broc Feeney and Jamie Whincup sitting 12th at the break with a 1.9s deficit.
Shane van Gisbergen and Richie Stanaway shared the #97 car, meanwhile, and finished the morning sitting 23rd.
There was good company at that end of the times, with Cam Waters just 21st and Chaz Mostert 22nd.
A second four-hour session kicks off at 1pm local time.
