2021: ATP & top 250 & Olympics singles & doubles debut

Klein won the doubles title at the 2021 Challenger La Manche with compatriot Alex Molčan 1–6, 7–5, [10–6].

Klein made his ATP main draw singles debut in March 2021 at the 2021 Argentina Open in Argentina, where he defeated Andrea Collarini, Thiago Seyboth Wild and Ernesto Escobedo to qualify for the main draw. Klein then went on to upset compatriot Andrej Martin in the first round defeating him 7–5, 6–3. He would go on to lose in the second round to top seed, local favourite and eventual champion Diego Schwartzman in straight sets.^[4]

He reached the top 250 and a career-high of World No. 248 in singles on 28 June 2021 after reaching the second round also as a qualifier at the 2021 Mallorca Championships by defeating 8th seed Dušan Lajović.

At the Olympics, Klein was entered as an alternate for Matteo Berrettini, who had withdrawn due to a thigh injury.^[5][6] He also participated in the doubles event partnering Filip Polášek where they reached the second round.

2022: Maiden Challenger title, Major & top 150 debut

In May, he won his first Challenger at the 2022 Saturn Oil Open in Troisdorf, Germany as a qualifier without dropping a set.^[7]

He qualified for the 2022 Wimbledon Championships making his Grand Slam debut.^[8] He lost to wildcard Liam Broady in five sets.

He reached his second Challenger final in Alicante and won the title. As a result, he moved more than 40 positions up to a new career-high in the top 180 on 10 October 2022. Following his third final in Ortisei, Italy he moved 17 positions up into the top 150 at No. 146 on 31 October 2022.

2023-24: Australian Open debut and first Major win

He qualified for and reached back-to-back finals at the 2023 Sparkassen ATP Challenger in Ortisei, Italy. He won his third Challenger title defeating Maks Kaśnikowski.

In January 2024, he qualified for the 2024 Brisbane International and defeated sixth seed Sebastián Báez for his first ATP win of the season and only third in his career. He qualified for the 2024 Australian Open making his debut at this Major and recorded his first Major win over Soon-woo Kwon.

Singles: 15 (9–6)

Result	W–L	Date	Tournament	Tier	Surface	Opponent	Score
Loss	0–1	Aug 2017	Slovakia F2, Piešťany	Futures	Clay	Grégoire Jacq	4–6, 6–3, 3–6
Loss	0–2	Aug 2017	Slovakia F3, Bratislava	Futures	Clay	Filip Horansky	4–6, 4–6
Loss	0–3	Oct 2017	Czech Republic F7, Jablonec nad Nisou	Futures	Carpet	Patrik Rikl	4–6, 2–6
Win	1–3	Mar 2018	Egypt F9, Sharm El Sheikh	Futures	Hard	Patrik Nema	2–6, 6–3, 6–3
Win	2–3	Mar 2019	M15 Sharm El Sheikh, Egypt	World Tennis Tour	Hard	Jacopo Berrettini	6–4, 6–4
Win	3–3	Apr 2019	M15 Sharm El Sheikh, Egypt	World Tennis Tour	Hard	Daniel Michalski	6–2, 6–3
Win	4–3	Apr 2019	M15 Sharm El Sheikh, Egypt	World Tennis Tour	Hard	Jakub Paul	6–4, 6–3
Loss	4–4	Jun 2019	M25 Pardubice, Czech Republic	World Tennis Tour	Clay	Manuel Guinard	4–6, 7–5, 6–7(6–8)
Win	5–4	Jul 2019	M15 Piešťany, Slovakia	World Tennis Tour	Clay	Fabian Marozsan	6–1, 4–6, 6–1
Win	6–4	Sep 2019	M15 Bratislava, Slovakia	World Tennis Tour	Clay	Kirill Kivattsev	7–6(7–3), 6–3
Win	7–4	May 2022	Troisdorf, Germany	Challenger	Clay	Zizou Bergs	6–1, 6–4
Win	8–4	Oct 2022	Alicante, Spain	Challenger	Hard	Nick Hardt	6–3, 6–4
Loss	8–5	Oct 2022	Ortisei, Italy	Challenger	Hard (i)	Borna Gojo	6–7(4–7), 3–6
Loss	8–6	Sep 2023	Istanbul, Turkey	Challenger	Hard	Damir Džumhur	6–7(5–7), 3–6
Win	9–6	Oct 2023	Ortisei, Italy	Challenger	Hard (i)	Maks Kaśnikowski	6–7(4–7), 7–6(7–4), 7–6(8–6)

Doubles: 11 (5–6)

Result	W–L	Date	Tournament	Tier	Surface	Partner	Opponents	Score
Win	1–0	Feb 2016	Turkey F6, Antalya	Futures	Hard	Alex Molčan	V. Alekseenko F. Ferreira Silva	7–6(9–7), 7–6(7–5)
Win	2–0	Oct 2016	Czech Republic F8, Jablonec nad Nisou	Futures	Carpet	Patrik Nema	Matej Vocel Pavel Motl	6–1, 6–3
Loss	2–1	Oct 2016	Czech Republic F9, Opava	Futures	Carpet	Patrik Nema	P. Matuszewski Grzegorz Panfil	1–6, 3–6
Win	3–1	Apr 2017	Greece F4, Heraklion	Futures	Hard	Patrik Nema	H. Callahan Nicholas S.Hu	6–4, 6–2
Loss	3–2	Jul 2017	Slovakia F1, Trnava	Futures	Clay	Patrik Nema	P. Kekercheni D. Kalenichenko	2–6, 7–5, [8–10]
Loss	3–3	Apr 2019	M15 Sharm El Sheikh, Egypt	World Tennis Tour	Hard	Marek Jaloviec	Michal Dembek Daniel Michalski	2–6, 6–3, [8–10]
Loss	3–4	Jun 2019	Bratislava, Slovakia	Challenger	Clay	Alex Molčan	Joran Vliegen Sander Gillé	2–6, 5–7
Win	4–4	Sep 2020	M15 Jablonec, Czech Republic	World Tennis Tour	Clay	Uladzimir Ignatik	Filip Duda Petr Nouza	6–3, 7–6(7–4)
Loss	4–5	Nov 2020	Bratislava, Slovakia	Challenger	Hard	Alex Molčan	Harri Heliövaara Emil Ruusuvuori	4–6, 3–6
Win	5–5	Feb 2021	Cherbourg, France	Challenger	Hard	Alex Molčan	Albano Olivetti Antoine Hoang	1–6, 7–5, [10–6]
Loss	5–6	Mar 2021	Zadar, Croatia	Challenger	Clay	Alex Molčan	Blaž Kavčič Blaž Rola	6–2, 3–6, [3–10]

Doubles: 1 (1 runner-up)

Result	Year	To	Surface	Partner	Opponent	Score
Runner-up	2016	Australian Open	Hard	Patrik Rikl	Alex de Minaur Blake Ellis	6–3, 5–7, [10–12]