Since 1997 he was international activ, at the US Open he won the silver medal in the doubles. 2001 he took part in the World Championships for the first time, where Joo won bronze with the South Korean Team. In the singles he lost early. In October he could push forward to place 50 in the world rankings. Till May he fell back to place 61, in this month he won the silver medal in the singles. In the final he lost to Werner Schlager and moved to place 22, whereby he became best chopper in the world. In October 2003 Joo took part in the World Cup for the first time, but retired in the group qualification. 2004 he reached the semifinals with the team at the World Championships, so they won bronze. At his first Olympics he reached in singles and doubles the round of 16, and at the World Cup he got place 4 after losing Wang Hao in the third place match. In 2005 he doesn't was really activ, so he lost his best defender title to Chen Weixing.