In March 2021, Kuzmanov reached his first ATP Challenger final at the Zadar Open in Croatia which he lost 6-2/2-6/6-7(5) to Serbian Nikola Milojevic. It was the longest Challenger final for the season registered at three hours and seven minutes. A week later, in April, he followed with a quarterfinal showing at the Open de Oeiras Challenger. Thanks to these great results, he reached his best singles ranking of World No. 237 in singles.
