K-1 is a kickboxing combat sport that combines standup techniques from karate, taekwondo, kickboxing and traditional boxing, among others. The name is a play off the abbreviation of Formula 1, which is F-1. The sport was first formed by Kazuyoshi Ishii, a former Kyokushin karate competition fighter who had formed his own organization, Seido-kaikan karate, in 1980. s to K-1 websites.

There is currently a 70.5kg (155lb) weight division in K-1 called K-1 MAX ("Middleweight Artistic Xtreme"). Sing has been fighting in this weight division in the February 2006 K-1 MAX event in Busan, Korea.