Kwak first auditioned for the 1999 campus drama KAIST, where he met screenwriter Song Ji-na. Song took an interest in the young upstart and wrote a new character for him, leading to his acting debut. Upon the suggestion of his then-manager, he began using the stage name Chae Ji-sung, then decided to drop the surname “Chae,” and became known simply as “Ji Sung.” After several minor roles, he began playing bigger parts, notably in the television dramas Wonderful Days (also known as Splendid Days, 2001) and Sunshine Hunting (also known as Days in the Sun, 2002), as well as the North-South romantic comedy film Whistling Princess (also known as The Hidden Princess, 2002).