2010-11: Dmitrii Sergeev played for Traktor Chelyabinsk's 1996-team which was the top team in Ural Region and played for the Russian U16 championship. He also was chosen to represent the Ural region in U15 play. Sergeev scored 14 goals with 19 assists and had 58 penalty minutes in 41 regular season games and scored 4 goals with 5 assists and 4 penalty minutes in six games at the Ural tournament in Magnitogorsk. Sergeev scored 3 goals with 4 assists and was -1 with 2 penalty minutes in six games at the U16 Russian Championships. Traktor finished fourth; falling in overtime to Neva St. Petersburg in the bronze medal game. In five games with the Ural Region U15 team Sergeev had 3 assists and was +7 with 4 penalty minutes.
2011-12: Sergeev skated for Avangard Omsk in the Russian U17 championship after playing for Traktor Chelyabinsk during the Ural Region season and played for Russia's U16 team in international play. In five games for Avangard in St. Petersburg he scored 2 goals with 1 assist and was +4 with 4 penalty minutes. Omsk won a silver medal after falling to Ak Bars Kazan in the championship game. In 34 games for the Traktor 1996's he scored 12 goals with 14 assists and 14 penalty minutes. Sergeev scored 1 goal with 3 assists and was +7 with 6 penalty minutes in six games for Russia's U16 team at the European Youth Winter Olympic Games and was +8 with 2 assists and 35 penalty minutes in six other games for the team.
2012-13: Sergeev played for Traktor Chelyabinsk's U17 team and represented Russia in three international tournaments including the 2013 U17 World Hockey Challenge in Canada. He scored 4 goals with 12 assists in 28 games in Ural Region play as Traktor advanced to the Russian championship; falling to Avangard Omsk in overtime in the third place game. Sergeev was -4 with no points and 2 penalty minutes in six games at the Russian championship. He had 1 assist and 24 penalty minutes in six games for Russia at the U17 WJC and in five other international games scored 1 goal with 1 assist and was +2 with 4 penalty minutes. Sergeev was selected by Traktor in the first round (31st overall) of the 2013 KHL Draft and was chosen by Kitchener in the first round (23rd overall) of the CHL Import Draft.
2013-14: Sergeev joined the OHL's Kitchener Rangers in his first season in North America and skated in 49 games; missing nearly a month early in the season with a hip injury. He scored 2 goals with 7 assists and was +5 with 22 penalty minutes. Kitchener finished last in the Midwest Division with the OHL's second-worst record and missed the playoffs. Sergeev was not among the 210 North American skaters in Central Scouting's final rankings but appeared in the International Scouting Service's Top 100 prospects prior to the 2014 NHL Draft.