After years competing in shoot style professional wrestling and occasional shoot fights, Han switched to
mixed martial arts when RINGS adopted the King of Kings (KOK) format in 1999, thus fully transitioning to an MMA promotion. By this time, Han was considered past his athletic prime. Despite this, Han amassed an impressive 8–1 record under RINGS KOK rules. Following a winning debut against RINGS rookie
Yasuhito Namekawa, Volk's skills were proven further against American wrestler and
Mark Coleman's teammate
Branden Lee Hinkle, 12 years his junior. Although Hinkle was able to take Han down and control him, the Russian grappler locked a tight
triangle armbar at the end of the first round and made him tap out. Han got a matchup closer in age and experience against Olympic freestyle alumni
Zaza Tkeshelashvili in a special Russia vs Georgia fight, but again Han won by submission with a
guillotine choke. In December, Han took part in the King of Kings tournament, where he was pitted in the first round against RINGS England leader
Lee Hasdell. The Russian took him down and landed effective strikes before securing position, and also attempted triangle chokes and
armbars, but Hasdell's defense was strong. Han resumed using unorthodox
ground and pound from the
knee-on-stomach position every time the fight hit the ground, but it was in a standing segment where he got the finish, overpowering the British kickboxer with combinations and injuring Hasdell's eye for a TKO victory. In the second round, Volk fought
Bobby Hoffman, who came to the bout with a 23–4 record. Hoffman controlled Han for the majority of the fight and tried to perform ground and pound, while Han worked in avoiding damage from the bottom. However, once in overtime, Han took Hoffman down in the corner and stood over him landing punches until the end of the time, winning the decision. During the semifinals, Han met young star
Antônio Rodrigo Nogueira, expert in
Brazilian jiu-jitsu and almost half of Volk's age. Despite his disadvantages, the Russian grappler was successful in stopping Nogueira from attempting submissions, even although Nogueira passed his guard and took his back at the end of the first round. When the second round arrived, Han
sprawled a takedown and gained his signature knee-on-stomach control, with the Brazilian coming back with a kneebar; the hold was completely extended, but Volk shockingly escaped. After a restart, Han came attacking with a
flying Kimura lock which Nogueira answered with an
omoplata/
ankle hold combination, but he was unable to submit Volk, and the match ended shortly after. Nogueira was given the unanimous decision, eliminating Han from the tournament. He has been out of competition since February 2002, but will return at the Lithuanian Bushido Association (Lithuania Bushido) convention on 20 November 2004. When Sergei Kharitonov participated in HERO'S on 17 September 2007, he participated as Han's team (CLUB VOLK HAN), and Han himself came to Japan. ==Martial arts instruction==