[2] Hanfmann made his ATP main-draw debut at the 2017 BMW Open after defeating Arthur De Greef and Uladzimir Ignatik in the qualifying rounds.

273, he upset both Gerald Melzer and Thomaz Bellucci to reach the quarterfinals,[4] where he lost to second seed Roberto Bautista Agut.

Hanfmann reached his second career ATP Tour final at the 2020 Generali Open Kitzbühel in Austria, but lost in straight sets to Serbian Miomir Kecmanović.

[7][8] At the Chile Open, he reached his ninth quarterfinal on clay and of his career as a qualifier defeating two Spaniards, defending champion Pedro Martínez and Roberto Carballés Baena.

[9] At the 2023 U.S. Men's Clay Court Championships, he reached his tenth clay-court quarterfinal as a qualifier defeating Yosuke Watanuki and second seed Tommy Paul, his biggest win in three years.

101 at the Italian Open on his debut at the tournament, also as a qualifier, he reached the third round defeating Nicolás Jarry and ninth seed Taylor Fritz for his first top-10 win of the season and only second in his career.

[15][16] As the second qualifying seed, he entered the main draw of the 2023 French Open as a lucky loser and defeated Thiago Monteiro in the first round in five sets for his first win at this major.

[17] At the next grass court tournament, the Mallorca Championships, he reached the semifinals defeating the top seed Stefanos Tsitsipas[18] and retiring wildcard Feliciano López.

[21] With his compatriot Dominik Koepfer he reached the semifinals of the 2024 Australian Open as an unseeded pair, in only his second participation in the doubles competition at this Major, and third overall at Grand Slams.