Preining slipped from second to fourth with a sluggish start but was able to fight his way back after the mid-race pitstop, overtaking polesitter Rast with 20 minutes to go to claim his first victory of the 2023 season.
Reigning champion Sheldon van der Linde rounded out the podium spots behind team-mate Rast, with 20kg of success ballast on his car preventing him from repeating his Saturday win.
The start of the race was delayed when several environment protestors caused a nuisance, with local police having to intervene to ensure the track was safe for racing.
When the action finally got underway, Rast led the field from pole position in his Schubert BMW, with van der Linde and Bernhard Porsche's Ayhancan Guven demoting front-row starter Preining to fourth.
Rast slowly but steadily began extending his lead at the front of the pack, building a 1.5s buffer within the first 15 minutes of the race.
When the pit window opened at the 20-minute mark, Preining was one of the first drivers to get his mandatory stop out of the way, an inspired call that laid the foundation for his victory.
With his tyres up to temperature, Preining was able to close the gap to van der Linde when the BMW driver completed his own pitstop a few laps later. Guven, who was previously running third ahead of Preining, had suffered a slow pitstop and had already dropped out of the battle for the podium by this point.
After several failed attempts, Preining finally found a way past the South African into Turn 1, sending his Porsche up the inside to snatch second position.
With the reigning champion out of the way, Preining set his sights on Rast, who was in a comfortable lead after completing his own pitstop.
But it took little time for the 24-year-old to close the gap to Rast, Preining getting on the tail of the three-time champion with 23 minutes of racing still left in the race.
After spending a few laps on the back of the BMW, Preining finally threw his car up the inside of Rast into Turn 1, riding over the kerbs and muscling his way past the German in order to take the lead of the race.
Rast complained about Preining's aggressive move over team radio but he had no opportunity to mount a counterattack, with the Porsche ace quickly extending his lead up to two seconds.
Although Rast managed to cut down the gap by half a second, Preining was never under threat and he eventually took the chequered flag with an extended margin of 2.181s to score his third career win.
Van der Linde finished half a second down on Rast in third, adding some crucial points to his championship tally after an up-and-down campaign until this weekend's Norisring event.
The best of the rest spot went to factory Lamborghini driver Mirko Bortolotti, who had a relatively uneventful race en route to fourth in his SSR Performance-run Huracan.
Abt Audi's Kelvin van der Linde passed the second Manthey EMA car of Dennis Olsen after his pitstop to finish fifth, while Laurin Henrich was seventh in the best of the Bernhard Porsches.
Henrich's team-mate Guven, who looked set for a podium in the first stint, was hit with a long-lap penalty for spinning his tyres while his car was still jacked-up, slipping to eighth place.
Landgraf Mercedes driver Maro Engel the Mercedes contingent in ninth, ahead of factory stablemates Luca Stolz (HRT) and Lucas Auer (Winward).
Franck Perera slipped from 10th on the grid to 12th in his SSR Performance Lamborghini, as Project 1 BMW's Marco Wittmann, Abt Audi's Ricardo Feller - the last driver to make a pitstop - and HRT Mercedes driver Arjun Maini completed the points scorers.
Race results:
| Cla | Nº | Driver | Car / Engine | Gap |
|---|---|---|---|---|
| 1 | 91 | Thomas Preining | Porsche | |
| 2 | 33 | René Rast | BMW | 2.181 |
| 3 | 1 | S.van der Linde | BMW | 2.612 |
| 4 | 92 | Mirko Bortolotti | Lamborghini | 3.453 |
| 5 | 3 | K.van der Linde | Audi | 3.852 |
| 6 | 90 | Dennis Olsen | Porsche | 5.063 |
| 7 | 75 | Laurin Heinrich | Porsche | 5.635 |
| 8 | 24 | Ayhancan Guven | Porsche | 8.046 |
| 9 | 48 | Maro Engel | Mercedes | 10.389 |
| 10 | 4 | Luca Stolz | Mercedes | 15.827 |
| 11 | 22 | Lucas Auer | Mercedes | 16.551 |
| 12 | 94 | Franck Perera | Lamborghini | 17.061 |
| 13 | 11 | Marco Wittmann | BMW | 17.829 |
| 14 | 7 | Ricardo Feller | Audi | 18.471 |
| 15 | 36 | Arjun Maini | Mercedes | 19.748 |
| 16 | 9 | Tim Heinemann | Porsche | 21.862 |
| 17 | 99 | C.Engelhart | Porsche | 23.049 |
| 18 | 84 | Jusuf Owega | Mercedes | 24.767 |
| 19 | 8 | Luca Engstler | Audi | 31.642 |
| 20 | 14 | Jack Aitken | Ferrari | 1 lap /30.886 |
| 21 | 69 | T.Vermeulen | Ferrari | 1 lap /40.241 |
| (22) | 6 | Alessio Deledda | Lamborghini | Retirement |
| (20) | 19 | Mick Wishofer | Lamborghini | Retirement |
| (24) | 83 | P.Niederhauser | Audi | Retirement |
| (25) | 63 | Clemens Schmid | Lamborghini | Retirement |
| (7) | 40 | Mattia Drudi | Audi | Retirement |
| (27) | 27 | David Schumacher | Mercedes | Retirement |
