Having secured a career-best third in qualifying earlier on Sunday in mixed weather conditions, Cassidy was able to pass Rast on track early in the race before leapfrogging van der Linde in the pits to claim the lead.
Van der Linde kept the pressure up on Cassidy for the remainder of the event, but the Kiwi was able to resist the advances of the Schubert BMW driver to become the 10th different driver to win a race this season.
At the start of the race, van der Linde made a rapid getaway from the outside of the front row to nose ahead of polesitter Rast, before completing the move on the run to Eau Rouge.
Rast, meanwhile, was forced to turn his attention to a charging Cassidy, who appeared quicker of the two in the first stint and made several attempts at passing the three-time champion for second.
On lap 7 Cassidy tried to make a move around the outside of La Source but got hit by the Abt Audi of Rast, who had moved to the right to take the racing line and would later be penalised with a five-second penalty for the offence.
Despite the contact, Cassidy was able to get a great exit out of the right-hander and could complete the pass, even as Rast tried his best to retake the position.
All of this allowed van der Linde to inch clear at the head of the field, but his hard work was undone by a slow 11-second pitstop just two laps later, as the Schubert pit crew struggled to remove the front-left tyre.
This meant that when Cassidy came into the pits two stops later and had his car serviced in just 7.6s, he was able to rejoin the field ahead of van der Linde in the net lead of the race.
Aware Cassidy was on cold tyres and at his most vulnerable, van der Linde immediately launched an attack on the Kiwi, who was forced to defend hard on the Kemmel straight.
The two drivers continued to battle into Les Combes and Bruxelles, with van der Linde appearing to have an edge at one point, but Cassidy somehow managed to keep his nose ahead to remain at the front.
Cassidy had pulled out a small lead when a full-course yellow had to be deployed for the stranded car of Rast, who had suffered an abrupt puncture while running in third place - but was yet to serve his penalty - in a major blow to his championship hopes.
The FCY period that Rast caused was followed by a one-lap dash to the finish, with drivers lining up in Indy-style formation for the restart.
As the race got going, van der Linde tried to take the outside line into La Source, but Cassidy had him covered and maintained the lead. The BMW driver had another look going into Les Combes, but the Kiwi again had him covered as took the chequered flag by 0.680s to celebrate his first win in the DTM.
It was also the second win for the AF Corse-run Red Bull operation in DTM this year following teammate Felipe Fraga’s victory at the Norisring in July.
Van der Linde had to settle for second position but with his Rast retiring and Mirko Bortolotti finishing only 10th, the South African was able to extend his championship lead even further with just two rounds still to go.
The final spot on the podium once again went to the Team Bernhard 75 Porsche of Thomas Preining, who was able to pass the Winward Mercedes of Lucas Auer and then resist the Austrian on the final lap.
Fourth place for Auer, however, means he climbs to second in the championship, ahead of the Audi of Rast.
Auer led a trio of Mercedes cars at the finish, with HRT”s Luca Stolz and GruppeM driver Mikael Greiner finishing right behind him in fifth and sixth respectively.
Schubert BMW’s Philipp Eng was classified seventh ahead of the second Red Bull of Fraga, as Marco Wittmann took advantage of the midfield chaos on the last lap to climb to ninth in his Walkenhorst BMW.
Bortolotti could only salvage a 10th place finish after starting 22nd on the grid, continuing his poor form in the second half of the season.
Spa DTM - Race 2 results
| Pos | Driver | Car | Gap |
|---|---|---|---|
| 1 | Nick Cassidy | Ferrari | |
| 2 | Sheldon van der Linde | BMW | 0.680 |
| 3 | Thomas Preining | Porsche | 1.673 |
| 4 | Lucas Auer | Mercedes | 2.756 |
| 5 | Luca Stolz | Mercedes | 3.013 |
| 6 | Mikael Grenier | Mercedes | 4.541 |
| 7 | Philipp Eng | BMW | 5.863 |
| 8 | Felipe Fraga | Ferrari | 6.565 |
| 9 | Marco Wittmann | BMW | 6.837 |
| 10 | Mirko Bortolotti | Lamborghini | 7.096 |
| 11 | Arjun Maini | Mercedes | 7.948 |
| 12 | Laurens Vanthoor | Porsche | 8.387 |
| 13 | Ricardo Feller | Audi | 8.821 |
| 14 | Kelvin van der Linde | Audi | 9.372 |
| 15 | Maximilian Götz | Mercedes | 9.735 |
| 16 | Marius Zug | Audi | 11.769 |
| 17 | Rolf Ineichen | Lamborghini | 15.768 |
| 18 | Alessio Deledda | Lamborghini | 17.150 |
| 19 | Dennis Olsen | Porsche | 17.239 |
| 20 | David Schumacher | Mercedes | 53.368 |
| 21 | Maximilian Buhk | Mercedes | 56.607 |
| 22 | Nico Muller | Audi | 1'09.200 |
| 23 | Maro Engel | Mercedes | 1 lap |
| 24 | Esteban Muth | BMW | 1 lap |
| 25 | Dev Gore | Audi | 1 lap |
| 26 | Clemens Schmid | Lamborghini | 3 laps |
| (3) | Rene Rast | Audi | Retirement |
