[4] Gassiev had his second fight in November at the Traktor Sport Palace in Chelyabinsk, Russia, defeating Vladimir Chuklin, knocking him out in round 4.

Over the next 18 months, Gassiev remained unbeaten with a record of 11 wins with 7 coming inside the distance at a tender age of 19.

In June 2013, Gassiev defeated 24 year old Levan Jomardashvili (31-10, 21 KOs) to claim the vacant WBC Youth world cruiserweight title at the Yunost Arena in Chelyabinsk.

[7] On 30 August 2014 Gassiev fought on the undercard of WBO cruiserweight title fight Huck-Larghetti at the Gerry Weber Stadium in Halle, Nordrhein-Westfalen, Germany.

Two months later, Gassiev won the vacant IBF Inter-Continental cruiserweight title after stopping Engin Karakaplan in round 1.

In January he made his debut in the United States, stopping Terrance Smith in round 4 before fighting his biggest challenge to date, former USBA and NABF cruiserweight champion Felix Cora Jr. (25-6-2, 14 KOs) at the Foxwoods Resort in Mashantucket, Connecticut.

[11] In June 2015, Gassiev knocked out Rodney Moore in 2 rounds and then fought the undefeated Isiah Thomas (15-0, 6 KOs) on 18 December 2015 at the Palms Casino Resort, in Las Vegas, Nevada in an IBF eliminator.

The fight ended after round 3 in a no contest after Thomas was hit accidentally after the bell and could no longer continue following the ringside doctor's orders.

[12] It was announced that Gassiev would get another opportunity in an eliminator against #10 IBF Jordan Shimmell (20-1, 16 KOs), whose only defeat came to Isiah Thomas.

[13] It was announced in August 2016, that a fight between Gassiev and unified cruiserweight world champion Denis Lebedev (29-2, 22 KOs) was in the works and a date was set for 3 December 2016 at the Megasport Arena in Moscow.

[20][21] It was announced on 8 September by Ringstar Sports that the fight would take place in Newark, New Jersey, at the Prudential Center on 21 October 2017.

[22][23] Gassiev delivered a dominant third-round KO, retaining his IBF title and securing his place in the semi-finals.

[24][25] Immediately after Gassiev's win over Włodarczyk, WBA 'Regular' champion Yuniel Dorticos (22-0, 21 KOs), who defeated Dmitry Kudryashov in September to advance to the semi-finals, entered the ring.

[28] On 1 February 2018, the WBA demoted Lebedev to 'champion in recess' and promoted Dorticos from 'regular' to 'full' champion in order for the fight with Gassiev to be a true unification.

[29] Gassiev dropped Dorticos three times in round 12 to score a dramatic win and advance to the final of the tournament.

The last knockdown drove Dorticos through the ropes, forcing referee Eddie Claudio to end the bout at 2 minutes and 52 seconds.

[34] After Gassiev defeated Dorticos, unified WBO and WBC cruiserweight champion Oleksandr Usyk (14-0, 11 KOs) entered the ring and congratulated him, setting up the final of the tournament, to take place in Jeddah, Saudi Arabia on 11 May 2018.

"[36] On 5 February, The Ring Magazine announced the winner would receive their vacant cruiserweight title as Usyk and Gassiev are ranked #1 and #2 respectively in their weight.

[37] After returning to North Ossetia, where he was met with 3,000 people, Gassiev stated he would be moving up to heavyweight after fighting Usyk.

[38] On 16 April, it was reported that Usyk had suffered an elbow injury during training, pushing the final to possibly June or July 2018.

[39][40] With the Russian Boxing Federation still pushing to host the final on 22 July in Moscow, Usyk replied that he would not be fighting Gassiev in Russia.

[43] On 18 June, at a press conference, Kremlev announced the final would take place on 21 July at the Olympic Stadium in Moscow, Russia.

After the fight, both combatants were exemplars of good sportsmanship, embracing, with Gassiev saying "I had the best opponent of my professional career ... today is Oleksandr's day".

Usyk used his superior conditioning to finish the fight, also increasing his output by landing 47 of 117 punches thrown in round 12.

[51] On 17 August 2018, it was reported that Gassiev would take time out to deal with an ongoing shoulder injury, potentially leading to surgery.

I want to make all my fans around the world happy and realize my full potential in the exciting Heavyweight division."