Undisputed II: Last Man Standing

Some years after fighting Monroe "Undisputed" Hutchens in prison, former boxing champion George "Iceman" Chambers visits the Russian Federation for a series of boxing matches, where he is subsequently framed for possession of cocaine and once again sent to prison. There, he discovers a series of illegal mixed martial arts matches dominated by inmate Yuri Boyka. The prison officials arrange these fights and place large side-bets to make a personal profit, often at the expense of the fighters. Chambers shares a cell with Steven Parker, a British junkie.
Mob boss Gaga and Warden Markov tell Chambers that if he fights Boyka, he will likely get an expedited appeal and early release. Chambers initially refuses, but after spending time in demeaning physical labor in the prison's sewer system and experiencing firsthand the brutality of the guards, he reluctantly agrees. He is rescued from both forms of humiliation by a wheelchair-bound inmate named Crot. Both fighters train hard for the match, though Chambers still relies on his boxing background while Boyka prepares a series of deadly kicks, throws, and grappling combinations designed to humiliate his opponent in the ring. Prior to the fight however, Boyka's gang force Steven, who is acting as Chambers' cornerman, to spike his water with a light sedative during the fight.
During the fight, Chambers is somewhat taken by surprise at the flurry and variety of Boyka's attacks, but manages to hold his own through the first round by keeping his distance, staying calm, and dealing out painful punching combinations whenever possible. At the end of the first round, Chambers takes a few sips of the spiked water, causing him to lose on the second round when he loses consciousness and falls to Boyka's flying kick. Following the match, Steven hangs himself out of guilt. Later, Chambers confronts Boyka over the sedative incident; after Boyka learns that his gang spiked Chambers' water behind his back, Chambers demands a rematch to ensure his release. Chambers receives training in mixed martial arts from Crot, who was a former soldier and the one who trained Boyka in his fighting style.
Once the rematch begins, it is apparent to all the viewers that the combatants are evenly matched and Chambers is more impressive in his style. The fight is long and intense, with flurries of combinations, grapples, and throws traded between the two. Eventually, Chambers realizes that Boyka will not lose consciousness, will not submit, and will likely knock him out if the fight goes on too long. Chambers alters his strategy and manages to get Boyka in a joint lock and ends the fight by breaking Boyka's leg, proving that he is the undisputed new champion of the prison. Shortly thereafter, Chambers is released from prison and uses his winnings to buy Crot's freedom as well. In a final scene, he wheels Crot to a train station to meet with his estranged niece in a happy reunion. Crot thanks Chambers for giving him the remainder of the winnings to start his life again, while Chambers expresses his gratitude for the help and training. Crot then meets with his long lost niece and the two embrace.

Question: Who is boxing?
Answer: Chambers and Boyka