"The basic difference was that today we scored the goals," said Internacional coach Celso Roth, whose team are the first from South America to miss out on the final after their 2-0 semi-final defeat to African champions TP Mazembe on Tuesday.
Forwards Tinga and Alecsandro scored before half an hour was up and the Asian champions were reduced to 10 men in the 34th minute when defender Jan Suk-won was sent off for a body-check on Tinga, his second booking.
Tinga stooped to Alecsandro's low right cross to head Internacional, winners of the tournament in 2006, into a 15th-minute lead.
The Libertadores Cup holders went further ahead when Alecsandro took a square pass from Argentine playmaker Andres D'Alessandro and shot inside the right-hand post.
Striker Dzenan Radoncic came closest for the South Korean side in the first half but goalkeeper Renan managed to block his shot and turn the ball away for a corner.
D'Alessandro made it three for the Brazilian side early in the second half with a fine left-footed shot from outside the box that curled away from keeper Jung Sung-ryong and went in off his left-hand post.
Alecsandro his second and Internacional's fourth in the 71st minute up before Colombian midfielder Mauricio Molina pulled two back for Seongnam in the final six minutes.
"If we'd had the luck to score (on Tuesday) our situation would have been different," Roth told the post-match news conference.
"If we're not playing for the cup today it's because at some moment we stepped over the limit," he said, agreeing that his team got carried away by their tag as favourites to reach the final with Inter Milan.
By beating the 2006 champions, Mazembe of DR Congo broke the monopoly Europe and South America have had on the tournament final.
That defeat was a bitter pill for Internacional as South American teams take the tournament more seriously than their European counterparts, regarding it as the pinnacle of club football.
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