Lee Sung-hyun first came to prominence by winning the KMAA Korean Welterweight Championship and debuted in K-1 on March 20, 2009, beating Kim Tae-hwan by unanimous decision at K-1 Award & MAX Korea 2009 in Seoul, South Korea. In his sophomore appearance in the promotion, he took another unanimous decision over Kizaemon Saiga at the K-1 World MAX 2010 in Seoul World Championship Tournament Final 16 on October 3, 2010. 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.
He was then recruited by Krush to compete in the tournament to crown the promotion's inaugural 63 kg/138 lb champion. 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. The bout was scored a draw after the regulation three rounds and so an extension round was needed to decide a winner, after which Urabe was awarded the unanimous decision. 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. This further established him as a top prospect in the region and he was soon employed to fight on the main RISE events headquartered in Tokyo. At RISE 85 on November 23, 2011, Lee's momentum was slowed down as he lost in a non-title bout against RISE Super Lightweight Champion Koji Yoshimoto via unanimous decision. Had he not been deducted a point before the start of the fight for missing the contracted weight, he would have gotten a majority draw.
Lee bounced back with an extension round points win over Park Byung-kyu at The Khan 3: New Generation in Seoul on January 15, 2012 before taking on another reigning RISE champion, lightweight titlist Yuki, in a 64 kg/141 lb non-title affair at RISE 88 on June 2, 2012, and winning a majority decision. 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. Although he was floored with a spinning back kick in round two, Lee controlled the rest of the bout and earned an extra round in which he dropped Watanabe twice, forcing the Referee to call off the bout. He was scheduled to fight Hiroshi Mizumachi at RISE 90 on October 25, 2012 in a #1 contender's bout for a shot at Yuki's lightweight belt, but Mizumachi pulled out two weeks before with an injury and was replaced by Shohei Asahara. Lee bettered Asahara on the judges' scorecards and took a unanimous decision. 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.
With this, he earned a rematch with Yuki and a shot at the RISE Lightweight (-63 kg/138.9 lb) Championship in the main event of the RISE 91/M-1 ~Infinity II~ co-promotion on January 6, 2013. The fight went very much like their first encounter, where Lee used his advantage in speed and combinations to score on Yuki, particularly with low kicks that damaged the more powerful striker and led to a stoppage in round four and captured Lee the title as well as the #4 place in the world lightweight rankings. 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. Having won by unanimous decision, he then went up against Zheng Zhao Yu in the semis and won via TKO with a front kick to the body in the second round after forcing a standing eight count earlier. Pongthong Jetsada awaited him in the final, and Lee outpointed the Thai to win a unanimous decision and take the tournament crown. He remained at -70 kg/154 lb for his next outing as he made his Glory debut against Yoshihiro Sato at Glory 8: Tokyo - 2013 65kg Slam on May 3, 2013. Giving up 12 cm/4 inches in height, Lee lost to Sato by unanimous decision in a close match. Lee dropped to 65 kg/143 lb to fight RISE's super lightweight title holder Yasuomi Soda in a non-championship bout at RISE 94 on July 19, 2013. Soda was shown a yellow card after low blowing Lee twice in round one but stormed back to win a majority decision and stop Lee's five-fight winning streak in the promotion. Having qualified for the tournament with his K-1 Korea win, Lee beat Charles François by unanimous decision after scoring a first round knockdown at the K-1 World MAX 2013 World Championship Tournament Final 16 in Majorca, Spain on September 14, 2013.
Lee was initially set to fight Andrei Kulebin at the K-1 World MAX 2013 World Championship Tournament Final in Pattaya, Thailand on July 26, 2014. The event was postponed due to the 2014 Thai coup d'état, however.
