[5] His biggest junior success came in 2021 at the US Open, where he reached the quarterfinals in singles and his first Grand Slam final in doubles with Viacheslav Bielinskyi, but lost 3–6, 7–5, [1–10] to Max Westphal and Coleman Wong in the championship match.

Earlier in the year the Bulgarian-Ukrainian duo came close to another doubles final at the French Open, but lost out in the semifinals to eventual champions Arthur Fils and Giovanni Mpetshi Perricard.

Although he lost his first singles match, Petr reached his debut ITF final in doubles with compatriot Simeon Terziev, but the Bulgarian duo couldn't go all the way, losing 3–6, 3–6 to Luca Gelhard and Kai Wehnelt.

In 2022 he made his debut for Bulgaria Davis Cup team in the World Group II Play-Off tie with Paraguay, but was forced to retire in the third set against Daniel Vallejo.

He also received a wildcard for the 2023 Sofia Open with Yanaki Milev and also reached the quarterfinals defeating Spanish duo Íñigo Cervantes and defending champion David Vega Hernández.