Even before the first laps had been completed at the Nurburgring, BMW was downbeat about its chances at the former German Grand Prix venue, with the boost pressure of the M4 GT3 having been significantly reduced compared to the previous rounds.
But the decision to go aggressive on strategy in both races made things worse for its most high-profile signing, with Rast failing to score points during an entire race weekend for only the second time in four seasons.
In the first race on Saturday, Rast was running in ninth place after completing his mandatory pitstop on lap 18 of 39. He hadn't been able to replicate the kind of pace that had seen his team-mate Sheldon van der Linde climb from 17th to seventh, primarily due to a set-up that worked over a single-lap but not in race trim, but he was still in for a solid haul of points.
However, Rast and his engineer Florian Rinkes then collectively decided to make an unscheduled second pitstop for wet tyres during a safety car phase, hoping that the rain will get heavier and will suddenly make the German a contender for the victory.
But contrary to their expectations, the rain stopped and the track dried out quickly, leaving Rast helpless on his grooved Pirelli tyres. The 36-year-old was left with no choice but to come in again for a third time and switch back to slicks, leaving him a distant 20th at the finish.
Things didn't look any brighter on Saturday as Rast could only qualify 12th on the grid in a wet qualifying session, while van der Linde managed to put his BMW on the front row.
Electing to start the race on slicks, Rast opted for a careful approach in the opening laps as the first sector of the track was still wet from earlier rain showers. However, just as the track began to dry out and Rast had started to lap quicker than the leaders, the heavens opened up, leaving him vulnerable on slicks.
Rast had to wait until the pitstop window opened after 20 minutes of racing before he could finally switch to wet tyres, but by then he had already lost too much ground to his rivals.
Although he showed impressive pace once his BMW was fitted with grooved Pirellis, Rast finished the race a lap down in 19th place, only ahead of Abt Audi's Kelvin van der Linde, who was a victim of a first-lap incident.
"It was hardly worth me travelling to the Nurburgring this time," said a dejected Rast. "That was a weekend to forget.
"We chose the wrong tyre strategy in both races, which meant we had no chance of scoring any points.
"It started to rain on Saturday and we switched to wet-weather tyres. Just as I seemed to be going faster, the rain stopped. It was exactly the opposite on Sunday.
"It is disappointing, but you have weekends like that sometimes."
Rast was always going to face a tough task in adding a fourth DTM title to his tally in 2023, having been forced to miss the Zandvoort round due to clashing commitments with McLaren in Formula E. And with half of the season now complete, the championship is looking increasingly out of his reach.
The double non-score at the Nurburgring has left him ninth in the standings, 63 points down on championship leader Thomas Preining (Porsche). And with team-mate van der Linde currently second to Preining but 28 points off the lead, Rast faces the prospect of having to support his team-mate's title bid.
