Inter will meet surprise packages TP Mazembe of DR Congo on Saturday after the African champions upset Internacional of Brazil 2-0 to become the first team from outside Europe or South America to reach the final.
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Dutch midfielder Sneijder pulled a thigh muscle when tackled in the second minute and was replaced by Thiago Motta in the fourth.
"To lose Sneijder early was not easy. We had to readjust," coach Rafael Benitez told reporters. "He has a pulled muscle, It's unlikely he'll get to the final."
However, a third-minute goal by midfielder Dejan Stankovic settled Inter into a counter-attacking game.
Samuel Eto'o crossed low from the left and defender Cho Byung-kuk, trying to stop Diego Milito from taking the ball, pushed it into the path of Stankovic who fired into the net.
Captain Javier Zanetti scored a fine second goal just past the half hour, racing into the box and taking a return backheeled pass from Milito to fire home.
The third came from Milito in the 73rd. Eto'o shot from the left, goalkeeper Jung Sung-ryong parried and the Argentina striker controlled with his chest and knocked the ball in as Seongnam called in vain for handball.
REFERENCE POINT
Benitez said it was good to have Milito available again after his absence and that of other injured players had made for a difficult early season for Inter following the glory of three trophies under Jose Mourinho last season.
"With Milito as the reference point for the attack it becomes easier for the others and for me," he told a news conference.
The Asian champions will play for third place against Internacional.
Seongnam came closest in the first half when tall Montenegrin striker Dzenan Radoncic's header from Colombian Mauricio Molina's free-kick was tipped over by Julio Cesar.
The Brazilian went down to a hard free-kick from Seongnam's Australian captain Sasa Ognenovski and Radoncic was left with an open goal but shot just wide.
Seongnam's coach Shin Tae-yong said Inter's winning margin did not worry him and he was happy with the way his team had played in attack.
"I'm not focusing on the goals but how we played. We could have defended better," he said.
"Inter are a level above us."
