He won his first ATP tour-level match as a lucky loser at the 2017 Antalya Open when he beat Víctor Estrella Burgos.

[3] Shoulder and hip injuries held back Altmaier's progress in his tennis career, but victories in qualifying over Tallon Griekspoor and Ruben Bemelmans led to his Grand Slam debut at the 2020 French Open, where he defeated Feliciano López, 30th seed Jan-Lennard Struff and 7th seed Matteo Berrettini, all of them in straight sets.

In October, Altmaier recorded his first win at a Masters 1000 level on his debut at the 2021 BNP Paribas Open defeating Sam Querrey.

[7] In April, Altmaier claimed his seventh Challenger title at the 2023 Sarasota Open defeating defending champion Daniel Elahi Galán.

He defeated compatriots Oscar Otte and Yannick Hanfmann to reach the fourth round for the first time in his career at the Masters level.

[9] He reached the quarterfinals for the first time defeating Jaume Munar in straight sets and moved 30 positions back up in the top 65 in the rankings.