South Korea coach Hong Myung-bo told his team to keep their heads up and channel their disappointment into sealing a spot in the World Cup knockout rounds after a 1-0 defeat to Mexico.
Goalkeeper Kim Seung-gyu dropped the ball to gift the co-hosts the only goal of a largely dull game on Thursday.
The narrow victory at a delirious Guadalajara stadium sent Mexico into the last 32 as Group A winners.
South Korea are still in a strong position to join them, sitting second in the group after beating the Czech Republic 2-1 in their opening match.
Hong's men face South Africa on Wednesday in Monterrey knowing that a draw will guarantee they advance.
"The result is quite disappointing but the players executed their plans well overall," Hong said, according to Yonhap News Agency.
"It's a shame we conceded the goal the way we did, but the players gave their best effort."
He added: "We don't need to hang our heads, no matter how disappointing this outcome is."
After a dreary first half in which South Korea had plenty of the ball without creating anything, Luis Romo's goal just after the break turned out to be the winner.
Goalkeeper Kim came out for a high ball, collided with his own defender and dropped it, leaving Romo with an empty net.
Kim, 35, who missed the birth of his first child because he was preparing for the World Cup, told reporters he lost concentration.
There was also suggestions of a miscommunication with Lee Gi-hyuk, the defender he bumped into on the way down before dropping the ball.
"My call might not have been heard clearly, and it was a situation where various factors were involved," said the goalkeeper.
South Korea will be favourites to defeat South Africa, who were beaten 2-0 by Mexico and rescued a late 1-1 draw with the Czech Republic on Thursday.
South Africa will be short of maximum strength because of suspensions, but Hong warned: "We can't be thinking about which key players won't play for them because it could make us feel complacent.
"I watched both of their matches so far and they have good speed. We have to be well organised against them."
--------------------------------
Where to watch the FIFA World Cup 2026™
Fans can watch every FIFA World Cup 2026™ match live on DStv Premium, Compact Plus, Compact, Family and Access. All matches are also available to stream on supported packages, ensuring flexible viewing options wherever you are.
How to stream the FIFA World Cup 2026™
With DStv Stream, you can follow every match anytime, anywhere. Download the app and enjoy seamless live streaming on the go.