The Asian champions, who won with goals by Mauricio Molina, Sasa Ognenovski, Choi Sung-kuk and Cho Dong-geon, will meet the Italians at the same Zayed Sports City stadium on Wednesday.
Colombian midfielder Molina gave Seongnam an early lead against Al-Wahda with a left-foot volley from the edge of the area. Brazilian striker Fernando Baiano equalised in the 27th minute. Ognenovski (30th), Choi (71st) and Cho (81st) put the result in favour of South Koreans.
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