From 1997 to 1998, Müller was a Formula One testdriver for Arrows and Sauber-Petronas, before joining the BMW-WilliamsF1 project to do tests for engines and Michelin tyres from 1999 to 2001.

The 2004 24 Hours Nürburgring was won by both Müllers (and Hans-Joachim Stuck) with the BMW M3 GTR V8 that had been raced successfully in the 2001 ALMS.

Müller also made a guest appearance at that year's Suzuka 1000 km, helping GSR & Studie to a 2nd-place finish, but was later disqualified during the post-race inspection.

While Müller and teammate Seiji Ara were unable to pull off a single victory in the season, despite several close battles for P1 on some occasions, their efforts were enough to land them in 3rd place in the overall standings.

Augusto Farfus helped Müller and Ara secure a podium at the 1000 km race in Suzuka.

However, a braking problem with the car resulting in a repair lasting at least two hours cost the team any chance of a class victory.

However, a pit fire with his car cost the team 7 minutes, and by the end of the race Müller finished in an impressive 4th place (the sister car finished 2nd), with fellow works drivers Augusto Farfus, Nicky Catsburg, and Dirk Adorf.

During the 24-hour race with Wittmann, Krohn, and Augusto Farfus, Müller was leading the field until his M6 GT3 suffered from a broken powertrain.

The car was subsequently sent to the Schubert garage for extensive repairs lasting over twelve hours and by the end of the race finished 120th overall.

The start of the 2017 season saw Müller join Schubert Motorsport again for an outing at the Dubai 24 Hour in a still-developing BMW M4 GT4.

The result was a flawless race for the team with the BMW M4 GT4 finishing 25th place overall (5th in the SPX class) having started from 49th on the grid.

The following month saw Müller join BMW Motorsport Juniors Ricky Collard, Nico Menzel, and Joel Eriksson as a mentor in a BMW M6 GT3 with Walkenhorst Motorsport for the Bathurst 12 Hour, but unfortunately was unable to participate in the race itself as the car crashed during qualifying.

The month after, it was announced Müller would rejoin BMW Team Studie again for a fourth season in the Super GT Championship.