Muller led the DTM standings for much of 2020 after starting the campaign with a run of three victories and two further triumphs in the middle of the year at the Nurburgring.
With three rounds to run, he sat 18 points clear of teammate Robin Frijns, with Rast 47 points adrift of him and practically out of the title fight.
But just as the fortunes of the Abt team declined in the Zolder double-header, Rast scored a run of four victories to turn the championship battle upside down.
This gave the 34-year-old a 19-point lead in the championship prior to the Hockenheim finale, which he successfully converted into a third title triumph in four seasons.
Muller said it “hurts” to lose the title after coming so close to upsetting the DTM’s benchmark driver, but felt he was as deserving of the championship as eventual winner Rast.
“Yeah, I’m disappointed still,” the Swiss driver said after the race. “We didn’t give up dreaming. We didn’t stop fighting until the very end, we kept believing in it.
"When the flag drops you realise it’s definitely over, the disappointment hits and it will take a night or two to digest that. An hour or two is not enough.
“We fought so hard for days and worked very hard for not only this whole season, but several years. To come that close and to really be in a position to deservedly win that championship when it slips through your hands in four five weekends, it hurts.
“That doesn’t mean Rene doesn’t deserve it. I want to congratulate him for this huge achievement, three titles in a competitive championship is very impressive - and he also deserves the third one. But I think we would have [deserved the title] as well.
“But we made one or two mistakes, once or twice too much bad luck playing in, and then just to put together an optimal season and we did.”
Muller lost what looked like a definite win in the fifth round at the Nurburgring when a sensor issue caused him to lose as much as 70bhp, while a tap from Jamie Green at the following weekend left him down in fifth in the next race at the same track.
Muller’s title hopes took a further hit in the Zolder double-header, where a performance deficit was compounded by an ill-timed safety car in Race 2 and an incident with the WRT Audi of Harrison Newey in the third race.
When Abt team boss Thomas Biermaier was asked where his driver Muller lost the title, he put the blame squarely on the events at Zolder.
“One thing is clear Rene is a good driver,” he said. “So if they have pure performance. I think we were not bad. Where we lost was Zolder clearly.
“I don’t want to mention it too many times, but we were also a bit unlucky on Nico’s side. In the Nurburgring, he did a great performance, he was leading the race, with the sensor failure he lost 15 points. The day before somebody turned us.
“After 18 races if somebody is leading the championship ranking the way he deserved it. But this year it’s especially hard because everybody in the team did a great job, fantastic job. We won the teams’ championship before.
“To be honest I have one driver on my left [Frijns] and the other one is on my right [Muller] and they both would deserve the title. They did a fantastic season. I have two champions next to me, we don’t have the trophy, but we we have two champions and they did a great job.
“But at the end we were not quick enough. I don’t know. It’s still too early to analyse it deeper. But we lost it in Zolder. Both Zolder weekends everything [came apart].”
Although both Muller and Frijns lost the drivers' title to Rast, Abt sealed the teams' championship with two rounds to spare at Zolder.