Daniel Altmaier

Altmaier made his ATP main draw debut as a qualifier at the 2017 Geneva Open, defeating Alexander Ward and Petr Michnev in qualifying. He lost to Sam Querrey in the first round.

He won his first ATP tour-level match as a lucky loser at the 2017 Antalya Open when he beat Víctor Estrella Burgos. In the next round, he defeated wildcard Marsel İlhan to reach the quarterfinals, where he lost to Yūichi Sugita. Both of his victories were decided by a third set tiebreak.

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 the fourth round, he lost to Pablo Carreño Busta.

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

Altmaier made his debut in the top 100 on 15 November 2021 at World No. 98 following the final at the 2021 Knoxville Challenger. He followed this by winning his third Challenger title for the year at the 2021 Puerto Vallarta Open.

He made his Grand Slam debut at the 2022 Australian Open and 2022 Wimbledon Championships.

At the 2022 US Open also on his debut, he took Jannik Sinner to five sets in his first round match.

In April, Altmaier claimed his seventh Challenger title at the 2023 Sarasota Open defeating defending champion Daniel Elahi Galán. He entered the 2023 Mutua Madrid Open as a lucky loser replacing 18th seed Pablo Carreño Busta directly into the second round, for only his second time at this round at a Masters 1000 level. He defeated compatriots Oscar Otte and Yannick Hanfmann to reach the fourth round for the first time in his career at the Masters level. 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. He lost to Borna Ćorić in straight sets. He continued his good form at the 2023 French Open where he reached again the third round, defeating eight seed Jannik Sinner in five sets in 5 hours and 26 minutes, the longest match since Lorenzo Giustino against Corentin Moutet six hours, 5 minutes win in 2020, the fifth longest overall of the tournament and the second longest of the season after Kokkinakis against Murray at the Australian Open.

He qualified for the 2023 Halle Open as a lucky loser but had to withdraw last minute due to hip injury.

At the 2023 US Open, he won his first main draw match at this Major defeating Constant Lestienne. As a result he reached the top 50 in the singles rankings on 11 September 2024. At the 2023 Paris Masters, he reached the third round after a walkover from Taylor Fritz who was injured.