The Japanese, preparing for their own opening game against Cameroon in Group E on Monday, had earlier arrived in chilly Bloemfontein, high on the central plains, plagued by doubts.
But their doubts were partially eased by Korea's performance, coach Takeshi Okada told reporters.
"This is very good motivation for us. Next it is our turn. They have shown us everything is possible," he said.
Worried about scoring and overall strategy, the Japanese put in a near two-hour training session at the Central University of Technology sports ground, focusing on corners and free-kicks.
Before that, the squad had watched the Koreans on television in their hotel.
Rivalry between the two Asian countries runs deep and when the Koreans beat Japan in a friendly last month, the Japanese were booed by their own fans for a lacklustre display.
Okada himself has been under fire from critics. He had set Japan a target of going through to the second round.
But although capable of a pleasing short-passing game, the Japanese have fared poorly in their warm-ups and struggled to score goals.
They face a powerful Cameroon in their opening Group E game on Monday, then take on the Netherlands and Denmark.
HARD TRAINING
Midfielder Makoto Hasebe acknowledged there were doubts about Japan heading into the tournament.
"We have had a hard week last week but what is important is the first game. We've had some hard training sessions and the team has a good feeling," the Wolfsburg player said.
His niggling back injury problem had cleared up, he added.
Team mate Junichi Inamoto said Cameroon would be a powerful opponent.
"Cameroon is not only about Eto'o," he said, referring to the African nation's striker and captain.
"They've got (Alex) Song and they are a very physical, attacking side. We will have to defend then go on the counter-attack," he told reporters after the training session.
"We have to win the first game. One point would be OK but three points better. The next game is against Holland and it is very difficult," he added.
