Petrov became age-eligible for the ISU Junior Grand Prix (JGP) series in the 2012-13 season and was assigned two JGP events in his debut season. He finished 5th in his JGP debut in Courchevel, France, and won the silver medal at his second JGP event in Turkey. A triple Axel was included in his programs for his first season on the ISU Junior Grand Prix circuit. This jump was landed and ratified in his second JGP event, albeit with a negative Grade of Execution (GOE). As the 2012-13 season progressed, Petrov went on to land several clean 3A with positive GOE, including one at the 4th stage of the Cup of Russia series in November 2012. Weeks later, at the 5th stage of the Cup of Russia series, Petrov landed a combined total of three clean 3A including a 3A-3T in the free skate. He won that competition with a score of 221.15. On the international scene, Petrov won gold medals in the junior events at the 2012 Coupe de Nice and the 2013 20th Volvo Open Cup. At the Russian Championships, he was tenth in his senior-level debut and won the bronze medal in the junior event. In his final competition of the season, Petrov won the junior gold medal at the 2013 Triglav Trophy with an overall score of 192.97 points, which included a 3A (1.00 GOE) in the short and 3A (2.00 GOE) and 3A-2T (1.00 GOE) in the free skate.