Early career Maslobojev made his professional debut against Steve Hooper in May 2007, when the two fought for the WKA Heavyweight Championship. The fight went into the tenth round, after which Sergej won by knockout. In his next fight, Maslobojev won the Baltic Muay Thai Light Heavyweight title, with a decision win over Kristaps Zile. After defeating
Ricardo van den Bos, he entered the 2011
KOK European Heavyweight Grand Prix. He scored a first-round knockout of Vasile Popovici in the quarterfinal and a third-round knockout of Martynas Knyzelis in the semifinal, and faced Mindaugas Sakalauskas in the finals. Maslobojev beat Sakalauskas by knockout in the fifth round. Maslobojev went on a 2–2 run over his next four fights, before challenging the reigning WKN Heavyweight champion
Vladimir Mineev at Fight Nights: Battle of Moscow 15. Mineev won the fight by majority decision.
King of Kings For his next fight, Maslobojev was scheduled to fight Lukasz Butkiewicz, at the KOK World GP 2014 in Gdańsk. He won the fight by a second-round knockout. He followed this up with a fight against Pavel Voronin, whom he beat by knockout as well. He would go on to win four of his next five fights, losing only to
Roman Kryklia, before fighting Robert Dorin for the
KOK Heavyweight Championship. Sergej won the fight by unanimous decision. After winning the KOK title, Maslobojev went on an eight fight, two year winning streak, during which he earned decision wins over Gokhan Gedik,
Oleg Pryimachov and Thomas Bridgewater, as well as stoppage wins over
Antonio Plazibat, Chaiyo Sitteacherjulun, Stanislav Voytenko,
Carlos Ulberg and Bruno Susano. This run culminated in a title fight with
Stéphane Susperregui for the
WAKO World K-1 Heavyweight title. Maslobojev won by a fourth-round TKO, after Susperregui's corner was forced to throw in the towel. This winning streak was ended by
Ondrej Hutnik at Yangames Fight Night 5, when the two of them fought for the WKN European Oriental Rules heavyweight title, with Hutnik winning a majority decision. In his next fight, Maslobojev was scheduled to fight Petros Vardakas. Maslobojev knocked Vardakas out in the second round.
ONE Championship Following a TKO victory against Noureddine Ajnaou at KOK`55 World Series, Maslobojev signed with
ONE Championship. He made his organizational debut at |
ONE Championship: Grit and Glory, in a rematch with
Antonio Plazibat. Maslobojev won the rematch by unanimous decision. In his second fight with ONE, he fought
Florent Kaouachi at
ONE Championship: Pursuit of Power, and won by unanimous decision. Moving away from ONE Championship, Maslobojev earned a unanimous decision against Rafael de Souza and a stoppage win against Fraser Weightman.
GLORY In the second part of 2019, Maslobojev signed with
GLORY. He was scheduled to make his debut against John King at
Glory 68. King later withdrew, and Maslobojev was rescheduled to fight Bahram Rajabzadeh at
Glory 69: Düsseldorf. He won the fight by unanimous decision. Maslobojev was next scheduled to fight a rematch with Fraser Weightman at
Glory 73. The fight fell through, as Weightman was unable to gain a visa to fight in China. He was next set to face the
K-1 heavyweight champion
Roel Mannaart at
Glory 75: Utrecht. Sergej won the fight by TKO, breaking Roel's jaw in the first round. Maslobojev was scheduled to fight
Luis Tavares at
Glory 78: Arnhem. He later withdrew from the bout with an injury. Maslobojev was scheduled to face
Donegi Abena at
Glory: Collision 3 on 23 October 2021. He won the fight by split decision. Maslobojev was scheduled to face Olivier Langlois-Ross at KoK Mega Battle on 20 November 2021. He won the fight by unanimous decision. Maslobojev was booked to face Nika Kulumbegashvili at KOK 99 Mega Series on 19 March 2022. He won the fight by unanimous decision.
GLORY Light heavyweight champion Maslobojev was expected to face the #1 ranked Glory light heavyweight contender
Luis Tavares for the vacant
Glory Light Heavyweight Championship at
Glory 81: Ben Saddik vs. Adegbuy 2 on 20 August 2022. The fight was cancelled on 10 August 2022, due to "reasons on the part of Tavares". Later that day, it was revealed that Tavares had tested positive for
performance enhancing drugs. Maslobojev was instead scheduled to face
Tarik Khbabez for the vacant title. Maslobojev won the fight by split decision. Maslobojev made his first title defense against the one-time Glory light heavyweight title challenger
Donegi Abena at
Glory 83 on February 11, 2023. He lost the fight by a fourth-round technical knockout. The ringside physician stopped the fight due to a cut on Maslobojev's right shin. Maslobojev faced
Rhys Brudenell at KOK 117 Mega Series in Vilnius. He won the fight by first-round knockout. Maslobojev faced
Bogdan Stoica in the quarterfinals of the
Glory Light Heavyweight Grand Prix, held on June 8, 2024, in
Rotterdam, Netherlands. He won the fight by a first-round technical knockout. Maslobojev next faced
Bahram Rajabzadeh in semifinals of the one-day tournament. He retired from the contest 20 seconds into the second round, after a headbutt from Rajabzadeh left him with a cut on his chin. Maslobojev faced Mohammed Hamdi at UTMA #9 on September 28, 2024. He won the fight by unanimous decision. Maslobojev faced
Ștefan Lătescu at
Glory Collision 7 on December 7, 2024. He won the fight by unanimous decision. Maslobojev faced
Jimmy Livinus at UTMA #11 on February 22, 2025. He won the fight by a second-round knockout. Maslobojev challenged the
Glory Light Heavyweight champion Tarik Khbabez at
Glory 100 on June 14, 2025. He won the fight by a fourth-round technical knockout.
Failed drug test On August 26, 2025, Maslobojev vacated the
Glory Light Heavyweight Championship following positive tests to a metabolite of stanozolol on two samples - collected out-of-competition and in-competition - coming from
Tarik Khbabez fight which took place at
Glory 100 on June 14, 2025. He received a 12-month suspension beginning on 14 June 2025.
UTMA Maslobojev faced Alexandru Burduja at UTMA #16 on January 31, 2026. He won the fight by a first-round technical knockout. Maslobojev faced Ignas Pauliukevičius at UTMA #18 on April 18, 2026. He won 3-0 by an unanimous decision. ==Titles and accomplishments==