South Korean coach Shin Tae-yong has admitted he made his players wear different-numbered shirts in recent friendlies to confuse opponents, who he says cannot tell them apart.
Only two players - Son Heung-min and captain Ki Sung-yueng - wore their usual shirts against Bolivia and Senegal earlier this month, while everybody else switched around.
Tae-yong admitted to this, when Sweden manager Janne Andersson apologised after claims one of his scouts spied on a closed South Korea training session.
"We switched them around, because we didn't want to show our opponents everything, and to try and confuse them," said Tae-yong.
"They might know a few of our players, but it is very difficult for Westerners to distinguish between Asians and that's why we did that.
"All coaches probably feel their opponents are always spying on them.
"I think it's perfectly natural that we all try to get as much information on each other as we can."
Andersson apologised for his scout watching the training session, saying he wasn't aware it was a closed session.
"He heard about a practice session, he didn't understand that it was a closed session, he didn't understand and he watched from a distance," said Andersson.
"It's very important we show respect to all our opponents in all circumstances. If someone could interpret it in another way, we regret it.
"It's been made a mountain out of a molehill."
The match between the two nations will be crucial, if either side are to qualify for the round of 16 from a difficult group, also featuring world champions Germany and Mexico.
Mexico stunned the defending champions 1-0 on Monday morning (NZ time).