Battlarts (1999–2000) Nagai turned his hand to professional wrestling and joined
Yuki Ishikawa's shoot style promotion
Battlarts. His run was somewhat successful, with Nagai earning victories over established wrestlers like
Takeshi Ono,
Yuki Ishikawa and
Katsumi Usuda. In the 2000 Young Generation Battle, Nagai went undefeated for the entirety of the tournament, beating Usuda,
Mohammed Yone, Mach Junji, Rastaman and
Minoru Tanaka to reach the final, where he lost to
Alexander Otsuka. After Battlarts went on hiatus beginning in November 2000, Nagai and the rest of the roster were forced to leave the promotion.
All Japan Pro Wrestling (2001–2005) Almost immediately after Battlarts began its hiatus, Nagai was announced for the 2000
Real World Tag League in
All Japan Pro Wrestling (AJPW), the promotion he had originally hoped to join in high school. Nagai was paired up with fellow UWF alumnus
Masahito Kakihara, calling themselves "Team Strongs". Despite the name, Team Strongs performed poorly in the tag league, earning just two points with a win over
Barry Windham and
Kendall Windham. In only his second match, Nagai lost to AJPW's top star
Toshiaki Kawada, and, even though he lost, he earned Kawada's respect and was selected to be his new tag partner. As a newcomer, Nagai was required to prove himself before he entered the 2001
Champion Carnival, and thus was put into the Champion Carnival Qualifying League. Nagai excelled in the qualification league which granted him entry to the Champion Carnival where he struggled, again earning just one win and two points. Despite mixed success and his newcomer status, Nagai was pushed as an All Japan loyalist and represented the company in interpromotional matches during AJPW's working relationship with
New Japan Pro-Wrestling (NJPW). On 6 June 2001, Nagai was involved in an incident during a match against NJPW wrestler
Takashi Iizuka at an NJPW show, where Nagai delivered a kick to Iizuka's face which resulted in Iizuka being severely concussed and needing over a year off to recover. Two days later when the NJPW roster came to the AJPW show in
Nippon Budokan, Nagai and Kakihara beat NJPW's
Yuji Nagata and
Shinya Makabe to win the vacant
All Asia Tag Team Championship, however, their reign was cut short after Kakihara suffered a knee injury and eventually left All Japan for NJPW after he had recovered, effectively breaking up Team Strongs. Nevertheless, Nagai's position as a loyalist in the
post-exodus era All Japan continued to rise, and he teamed with Toshiaki Kawada in a loss to
TenCozy (
Hiroyoshi Tenzan and
Satoshi Kojima) on 16 September on an NJPW card. Nagai teamed up with Kawada again for the 2001 Real World Tag League, seeing much greater success than the previous year, earning 10 points and making it to the final where they lost to
Keiji Mutoh and
Taiyo Kea. In early 2002, he entered the
Giant Baba Memorial Cup, a tournament focused on young wrestlers where he dominated, earning 24 points and beating Nobutaka Araya in the final to win the tournament. Not long after winning, he entered the 2002 Champion Carnival and performed respectably, earning 6 points but failing to reach the final. In what would be his final tour with All Japan, Nagai partnered with
Yoji Anjo for the 2002 Real World Tag League, earning 4 points and failing to reach the final. After 2 years with the promotion, Nagai left All Japan in December 2002.
New Japan Pro-Wrestling (2003–2005) Makai Club (2003–2004) Immediately after leaving AJPW, Nagai was signed by
New Japan Pro-Wrestling (NJPW), whom he had previously worked for during the inter-promotional relationship between both promotions. Due to his background in kickboxing and Rings, Nagai was placed into the
Makai Club, a group of wrestlers with legitimate backgrounds in martial arts. Nagai debuted under a mask as
Makai #5 at
Wrestling World 2003, teaming with the returning
Katsuyori Shibata, who was now masked and competing as Makai #4. In their debut as a team, the two defeated Nagai's former partner
Masahito Kakihara and
Takashi Iizuka. After defeating Kakihara in singles action at
Ryogoku Kokugikan in February, Makai #5 voluntarily unmasked himself as Nagai, though he continued to compete as both himself and Makai #5 after this. In July, Makai #5 and #4, who had dubbed themselves , challenged for the
IWGP Tag Team Championship, losing to reigning champions
Hiroshi Tanahashi and
Yutaka Yoshie in Osaka. Nagai competed at
Wrestling World 2004, teaming with
Makai #1,
Ryushi Yanagisawa and Ryota Chikuzen to defeat the Crazy Dogs (
Enson Inoue,
Hiro Saito,
Michiyoshi Ohara and
Tatsutoshi Goto). Nagai would compete twice at NJPW's King of Sports pay-per-view in March, first teaming with Shibata, #1 and Yanagisawa to defeat
Blue Wolf,
Shinya Makabe,
Toru Yano and Yutaka Yoshie, but later losing to
Josh Barnett in a singles match.
Face turn and team with Naruse (2004–2006) After the Makai Club broke up in the summer of 2004, Nagai began a face turn, returning to AJPW for one night only on 22 July where he lost to his mentor
Toshiaki Kawada. It was also during this time that he aligned himself with fellow Rings alumni
Masayuki Naruse, with the two unsuccessfully challenging
Genichiro Tenryu and
Masanobu Fuchi for the
All Asia Tag Team Championship on 26 July. Around this time, Nagai joined
Black New Japan while keeping his team with Naruse, which caused friction between them. On 3 November at
Masahiro Chono's 20th Anniversary Show, they challenged once again, this time beating Fuchi and Tenryu to win the titles. He also returned to AJPW in December, teaming with Kawada for the Real World Tag League. They made it to the playoffs, where a loss to a
RO&D (
Jamal and
Taiyo Kea) stopped them from reaching the final. After dropping the All Asia belts in February, Nagai suffered an injury competing against Naruse in March which would keep him out of action for the rest of 2005. In January 2006, Nagai was one of a number of New Japan wrestlers who opted not to renew their contracts with the promotion and became a freelancer.
Freelancing (2006–present) After leaving New Japan, Nagai debuted for
Dramatic Dream Team (DDT), choosing a lighter schedule in a comedy promotion which would allow his injury to heal at a quicker rate. He aligned himself with
Poison Sawada Julie's Serpent Council in late 2006, and also began competing for
Tatsumi Fujinami's
Muga World Pro Wrestling around this time. It was in MUGA Nagai would find his new home, competing regularly for the next few years while also making occasional appearances in DDT and various shoot style promotions such as
Battlarts and
Daisuke Ikeda's Fu-Ten. In February 2009, he wrestled
Canadian wrestler
Test in what would be Test's last match before he died the following month. In September 2010, he beat
Alexander Otsuka to win
Real Japan Pro Wrestling (RJPW)'s Legend Championship. He held the title for nearly a year before dropping it to
Super Tiger in July 2011. Nagai won it back from Tiger in March 2012, and again dropped it to Tiger in December. In January 2014, Nagai's appearances in Dradition became more sporadic, and he instead became a regular in
All Japan Pro Wrestling (AJPW) once again, forming the Dark Kingdom stable with
Kenso, which would later add
Black Tiger VII,
Black Tiger and
Takeshi Minamino to its ranks. As Dark Kingdom, Nagai and Minamino won the
All Asia Tag Team Championship in January 2015. They dropped the titles to
Ultimo Dragon and
Yoshinobu Kanemaru in March. Nagai would later debut and make
Pro Wrestling Noah his home in late 2017, entering the 2017 Global League where a loss to
Naomichi Marufuji stopped him from reaching the final. ==Fighting style==