He has long enjoyed skiing, cycling and swimming and remembers first touching a tennis racquet at three.

His grandmother, who competed on a national level, taught his older sister the game, so naturally he wanted to play.

[1] Lehečka qualified for the main draw of the 2022 Australian Open, defeating Michael Mmoh, Max Purcell, and Dmitry Popko.

[1] At the 2022 ABN AMRO World Tennis Tournament in Rotterdam, he reached the second round as a qualifier, with a stunning upset over fifth seed Denis Shapovalov in straight sets in his first ATP Tour main-draw match.

[14] He defeated Dominic Stricker in the semifinal but lost to Brandon Nakashima in the final in straight sets.

[19][20] Next he defeated sixth seed Félix Auger-Aliassime, his first top-10 win, to reach a Major quarterfinal for the first time in his career.

[22] At the same tournament he also reached the third round in doubles on his debut at the event at this Major with partner Alex Molčan.

[1] At the 2023 Qatar ExxonMobil Open he recorded his first top-5 win over top seed Andrey Rublev to reach the semifinals,[23] having previously defeated qualifier Damir Džumhur and Emil Ruusuvuori.

[26][27] In Monte Carlo he defeated Emil Ruusuvuori and Grigor Dimitrov to move to the round of 16.

[29] He entered the 2023 Croatia Open Umag as the top seed at a tour-level event for the first time, and reached the quarterfinals with a win over Dominic Thiem.

[42] Lehečka was runner-up at the European Open in Antwerp, Belgium, in October, losing to Roberto Bautista Agut in the final.

[43] At the 2025 Brisbane International he won his second ATP Tour title after the retirement of Reilly Opelka in the final.

[45] He made his debut at the 2019 Davis Cup qualifying round against Robin Haase of the Netherlands.