After winning a third national amateur title in 2007, she decided to turn pro as women's boxing was not yet accepted as an Olympic sport.
[2] In her next fight, Koseki moved up two weight classes to challenge for the WBC light flyweight title, but lost again by unanimous decision.
In August 2014, Koseki made her fourteenth consecutive defense, beating Denise Castle with a round 8 technical knockout.
With the win, Koseki broke Yoko Gushiken's Japanese national record of 13 consecutive successful world title defenses.
[4] In October 2015, Koseki beat WBA champion Ayaka Miyao by unanimous decision to make her sixteenth defense and unify titles.