On August 13, 2005 Ruslan Karaev made his K-1 tournament debut at K-1 World GP 2005 in Las Vegas as a virtual unknown.

In the semifinals, Karaev faced muay thai stylist Azem Maksutaj, what was later acknowledged as one of the greatest battles of K-1 history.

He topped his performance of the night by a unanimous decision win over American Scott Lighty and was crowned the tournament champion.

Ruslan missed his announced fight against Jerome Le Banner on September 29, 2007 at K-1 World GP 2007 final elimination in Seoul, Korea, due to a traffic accident suffered several days before the match.

[3] At the K-1 World Grand Prix 2009 Final 16 Karaev beat the new and current heavyweight champion, Keijiro Maeda by unanimous decision.