Rast and Frijns were due to race for the WRT squad, with Rast pencilled in to drive the team's #31 Audi R8 LMS alongside Kelvin van der Linde and Mirko Bortolotti and Frijns due to share the sister #30 machine with Christopher Mies and Dries Vanthoor.
Muller meanwhile was slated to join Sainteloc Racing alongside Christopher Haase and Markus Winkelhock.
A brief statement released by Audi on Tuesday morning said: "Together with Audi Sport, all three have decided to fully focus on the championship decision shortly before the DTM finale at Hockenheim."
On Wednesday, it was announced that Mirko Bortolotti will also skip the event due to a flu-like infection, prompting a major reshuffle in Audi's line-up.
WRT's DTM driver Ferdinand Habsburg, Dennis Marschall and Matthieu Vaxiviere will now share the team's #30 entry, with Mies and Vanthoor moved to the #31 car alongside van der Linde.
Dorian Boccolacci, meanwhile, will replace Muller in Sainteloc Racing's #25 Audi line-up alongside Haase and Winkelhock.
Vaxiviere made an outing with WRT earlier this year in the GTWCE Endurance Cup opener at Imola, taking victory alongside van der Linde and Bortolotti (pictured below).
Boccolacci has also partnered Haase and Winkelhock at Sainteloc for the first two rounds of the four-event campaign at Imola and the Nurburgring, while Marschall is a regular of the German-based ADAC GT Masters series with another Audi customer, Rutronik Racing.
Habsburg scored WRT's first podium and pole position in the DTM at Zolder last weekend.