Stefan Kozlov

Kozlov in 2016

In 2013, Kozlov made his ATP World Tour debut as a wildcard at the Newport tournament and lost in the first round. At the age of 16, he defeated his first Top 100 player in Tim Smyczek to reach his first ATP Challenger final in 2014 in Sacramento, where he lost to Sam Querrey. He also reached seven career ITF Futures finals and won four of these finals.

In April, Kozlov reached the final at the Open de Guadeloupe. In June, he won his first career ATP level match as a wildcard at the 2016 Ricoh Open in Den Bosch. He reached his first career quarterfinal by defeating Steve Johnson in the second round. Toward the end of the season, Kozlov reached a second Challenger final on the year at Suzhou. He won his first career Challenger event in Columbus. He won his second title at the 2017 Las Vegas Challenger.

In September, at the 2021 Columbus Challenger, Kozlov won his first title in four years, defeating Max Purcell in the final in three sets.[7] At the same tournament he won also in doubles partnering Canadian Peter Polansky. He followed this by a final at the 2021 Las Vegas Challenger and a second Challenger title for the season at the 2021 Charlottesville Men's Pro Challenger, defeating another Australian Aleksandar Vukic. As a result, he returned to the top 200 in more than three years at No. 188 on November 8, 2021.[8] He ended the year with winning his third Challenger in Champaign, US. As a result he finished the year ranked World No. 159 on 22 November 2021.

At the 2022 Australian Open he made his debut as a wildcard and won his first Grand Slam match against Jiří Veselý. He lost in the second round to 7th seed Matteo Berrettini.[9]

As a qualifier at the 2022 Delray Beach Open, he reached his second career quarterfinal after defeating Emilio Gómez[10] and Steve Johnson.[11]

Ranked No. 130 at the 2022 Abierto Mexicano Telcel in Acapulco, he reached the main draw as lucky loser and defeated Grigor Dimitrov in a stunning upset in three sets after fighting cramps, in the longest match in Acapulco history. It was the biggest win in his career.[12][13]

At the 2022 Wimbledon Championships, he made his debut at this Major[14] when he entered the main draw as a lucky loser after the late withdrawal of Borna Ćorić.[15] Kozlov lost in straight sets to 12th seed Diego Schwartzman. He reached a new career high ranking of No. 103 on 18 July 2022.