At the 2021 Australian Open Müller won his first Grand Slam match as a lucky loser defeating Juan Ignacio Londero.

At the 2023 Qatar ExxonMobil Open in Doha, Müller as a qualifier reached his first ATP quarterfinal in his career, defeating Nikoloz Basilashvili and eighth seed Botic van de Zandschulp.

[6] At the 2023 Grand Prix Hassan II he reached a second quarterfinal defeating sixth seed Richard Gasquet and Francesco Passaro.

[7] He then faced Russian Pavel Kotov, defeating him also in two hours in straight sets to reach his first ATP final, before losing to Roberto Carballes Baena.

[8] He made his Masters debut at the 2023 Italian Open (tennis) as a qualifier and recorded his first win defeating Kyle Edmund.

[citation needed] In February, Muller lost to Andy Murray in the first round of the ATP Qatar Open.

[18] In July, he won his first Challenger title in more than a year in San Marino, defeating Tseng Chun-hsin in the final.

[19][20] Müller received a wildcard for the main draw of the US Open,[21] where he reached the second round, recording his first win at the tournament over Adam Walton.

[22] In January, Müller won his maiden ATP title at the Hong Kong Open defeating Kei Nishikori in the final.