Known for his speed and sophisticated combinations, Lee debuted in K-1 in March 2009 and further established himself by winning the RISE Korea Super Lightweight Championship in June 2011.
After a slow opening round, Lee began to pull ahead in the second as he dropped Saiga twice in quick succession; firstly with a seven strike combination that culminated in a liver punch and then with a left hook and right low kick combo.
[5] At the Krush First Generation King Tournaments ~Round.2~ in Tokyo, Japan, on January 9, 2011, he fought Koya Urabe at the quarter-final stage.
[6][7] Lee returned to his home country and won a four-man tournament in Seoul on June 17, 2011, beating both Son Jun-hyuk and Park Don-fa by unanimous decision to be crowned the RISE Korea Super Lightweight (-65 kg/143.3 lb) Champion.
[12][13] In the main event of RISE 89 on July 1, 2012, Lee Sung-hyun was able to beat Yuto Watanabe by technical knockout in an extension round.
[16] At the RISE and M-1 Muaythai Challenge co-promoted event ~Infinity I~ on December 2, 2012, he knocked out Buakaw Weerasakreck with a right cross towards the end of the first round in a 63.5 kg/140 lb bout.
[21] He then moved up to -70 kg/154 lb and returned to K-1 after a two-year absence to compete in the K-1 Korea MAX 2013 eight man tournament in Seoul on February 2, 2013.
Drawn against Shingo Garyu in the quarter-finals, he scored a low kick knockdown over his Japanese opponent in round two before completely taking over in three.
Lee dropped him again with a right cross after hurting him with a body shot and at times simply teed off on Garyu who was content to block punches with his face.
Pongthong Jetsada awaited him in the final, and Lee outpointed the Thai to win a unanimous decision and take the tournament crown.
[44] Lee was scheduled to face Johnny Smith for the WKN World welterweight title at Prokick: Knockdown Lockdown on November 27, 2021.
[54] Lee faced Berjan Peposhi in the quarterfinals of the GLORY RISE Featherweight Grand Prix, held on December 21, 2024, in Chiba, Japan.