The least fancied of three formidable Koreans in the draw, Chang upset champion Ki in a nerve-jangling semi-final and then held off Unruh to ensure the title would remain in South Korean hands for the eighth time in nine Olympics.
It was Chang's second gold of the Games after earlier taking the women's team title with Ki and Choi-Misun.
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Uncharacteristically wayward in the late matches, Ki recovered her aim when it counted on a blustery day at the Sambadrome to defeat Mexico's Alejandre Valencia 6-4 in the bronze playoff and claim her fourth Olympic medal.
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