2017-2018: ATP debut, First ATP win Koepfer made his ATP main draw debut at the
2017 Winston-Salem Open as a
lucky loser. He won his first
ATP Challenger Tour title in doubles at the
Columbus Challenger, partnering
Denis Kudla. Koepfer won his first match on the
ATP Tour, again as a
lucky loser at the
2018 Winston-Salem Open, defeating
Tennys Sandgren.
2019: First Challenger title, Major debut and fourth round, top 100 After winning his maiden Challenger title in singles at the
Ilkley Trophy, Koepfer earned a wild card into the
Wimbledon Championships. There, he won his first Grand Slam main draw match by defeating
Filip Krajinović in the first round before losing to
Diego Schwartzman in straight sets. As a qualifier at the
US Open, Koepfer defeated 17th seed
Nikoloz Basilashvili in the third round, before losing to eventual finalist
Daniil Medvedev in four sets in the fourth round. As a result he reached the top 100 at world No. 86 on 9 September 2019. At the
Australian Open, he was defeated in the second round by third seed and last year finalist,
Dominic Thiem. Seeded sixth at the
Córdoba Open, Koepfer was eliminated in the first round by
Federico Coria. In
Buenos Aires, he fell in the second round to fifth seed
Albert Ramos Viñolas. At the
Mexican Open, he reached his first ATP tour semifinal beating Mexican wildcard
Gerardo López Villaseñor, fourth seed
Milos Raonic, and
Cameron Norrie. He ended up losing to second seed, compatriot, and eventual champion,
Alexander Zverev. With this result, Koepfer climbed into the top 60 in the rankings to a career-high of No. 54 in singles on 22 March 2021. At the
Miami Open, he was beaten in the first round by
Hugo Gaston. Koepfer began the clay-court season at the
Monte-Carlo Masters. Getting past qualifying, he lost in the first round to fellow qualifier
Marco Cecchinato. Next, he competed at the
Barcelona Open. He was defeated in the first round by fellow leftie
Corentin Moutet. Playing in
Munich, he was eliminated in the second round by seventh seed, compatriot, and eventual finalist
Jan-Lennard Struff. At the
Madrid Open, despite a second-round loss to 16th seed
Cristian Garín, he reached the top 50 following the tournament on 10 May 2021. His final tournament before the second Major of the year was the
Geneva Open where he upset seventh seed
Benoît Paire in a three-set victory. He ended up losing in the quarterfinals to third seed and eventual champion,
Casper Ruud. At the
French Open, he reached the third round for the first time and was defeated by eighth seed, former world number one, and 2009 champion
Roger Federer. Koepfer opened his grass-court season at the
Stuttgart Open. He lost in the first round to
Jurij Rodionov. In
Halle, he faced third seed Alexander Zverev in the first round. He pushed Zverev to three sets but ended up losing the match. At
Wimbledon, he made it to the third round for the first time where he fell to eighth seed Roberto Bautista Agut. After Wimbledon, Koepfer played at the
Hamburg Open. He was eliminated in the second round by top seed
Stefanos Tsitsipas. Representing Germany at the
Summer Olympics, he reached the third round and lost to sixth seed and eventual bronze medalist,
Pablo Carreño Busta. Starting his US Open preparation at the
Canadian Open, Koepfer was defeated in the first round of qualifying by Canadian wildcard
Peter Polansky. In
Cincinnati, he fell in the final round of qualifying to
Marcos Giron before earning a spot in the main draw as a lucky lower. He was defeated in the second round by seventh seed Pablo Carreño Busta. Seeded 16th at the
Winston-Salem Open, he reached the third round where he was beaten by top seed Carreño Busta. At the
US Open, he lost in the second round to second seed and eventual champion,
Daniil Medvedev. Competing at the first edition of the
San Diego Open, Koepfer was defeated in the first round by eventual finalist Cameron Norrie. At the
Indian Wells Masters, he fell in the first round to
Emil Ruusuvuori in three sets. In
Vienna, he was eliminated in the final round of qualifying by
Gianluca Mager. Entering the main draw as a lucky loser, he lost in the first round to
Lorenzo Sonego. In
Paris, he lost in the final round of qualifying to
Miomir Kecmanović but due to the withdrawal of
Jenson Brooksby, he entered the main draw as a lucky loser and defeated
Andy Murray in the first round, saving seven match points. His victory was named one of the top-5 comebacks of the 2021 season. In the second round, he upset ninth seed
Félix Auger-Aliassime. With this win, he ended the Canadian's hopes of qualifying for the
ATP finals. In the third round, he lost to seventh seed
Hubert Hurkacz in three sets.
2022: Loss of form, out of clay season and of top 200 At the
Australian Open, Koepfer defeated
Carlos Taberner before losing to 23rd seed
Reilly Opelka in the second round. Due to an ongoing arm injury, he decided to skip the entire clay season. As a result, his ranking plummeted to No. 149 on 18 July 2022, out of the top 150 at No. 159 on 22 August 2022, and out of the top 250 at No. 259 on 7 November 2022. The following week, he reached a second final at the
San Luis Open Challenger. As a result, he moved another 25 positions into the top 155. He also made the final at the
2023 Prague Open and reached the top 150 on 8 May 2023. Koepfer reached the semifinals at the
Los Cabos Open where he lost to fifth seed
Alex de Minaur. At the same tournament, he also reached his maiden ATP final with
Andrew Harris. At the
2023 US Open Koepfer lost to world No. 1 and defending champion
Carlos Alcaraz after retiring due to injury, having rolled his ankle.
2024-2025: Major doubles semifinals, Masters fourth round, injury, hiatus He won his fifth Challenger at the
2024 Canberra Tennis International. He then competed in the
2024 Australian Open, beginning his campaign against fellow compatriot and sixth seed
Alexander Zverev, falling in four tight sets. In doubles, alongside his compatriot
Yannick Hanfmann he reached the semifinals as an unseeded pair, before losing to Italian duo
Simone Bolelli and
Andrea Vavassori. At the
2024 Dallas Open he reached the quarterfinals defeating
Aleksandar Kovacevic and
Rinky Hijikata. He lost to eventual champion
Tommy Paul. He reached a second quarterfinal for the season at the
2024 Abierto Mexicano Telcel in Acapulco, defeating qualifier
Térence Atmane and eight seed
Frances Tiafoe. He lost to second seed
Holger Rune. He reached a new career-high ranking of No. 49 on 4 March 2024. ==Performance timelines==