Scoring goals has not been a problem for Suwon this season, they are joint top scorers with K League leaders Jeonbuk, but they have been prone to lapses in defence and that lack of concentration cost them on Tuesday.
Yeom Ki-hoon got the hosts off to the best possible start by putting Suwon 1-0 up in the second minute, but Akimi Barada was left unmarked to equalise 10 minutes later after Leandro's clever ball split the Bluewings defence.
Kashiwa were awarded a penalty after a careless challenge from Yang Sang-min left Kosuke Taketomi in a heap and Leandro put his side ahead from the spot.
Leandro looked to have killed off the tie with a goal 10 minutes into the second half when Suwon failed to cut out Cristiano's cross, and while North Korean striker Jong Tae-se pulled one back late on they will need a special performance in Japan next week to turn the tie around.
"It was a very disappointing night," said Seo. "We started so well with a very early goal and were feeling good.
"The first and second goals we conceded, however, were the result of defensive inadequacies. Our desire was not bad, but the problems came in defence.
"Even in situations that were not threatening, we gave goals away," added Seo.
"Jong and Yeom Ki-hoon played well and they combined excellently for the first goal and the second goal too," he said.
"We know that we have to attack in the second leg and the team selection will reflect that. We will be positive and aggressive and do everything possible to get the result that we need."

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