Highlights - Japan 1-0 Cameroon
Cameroon coach Paul Le Guen blamed his players' attitude for the 1-0 defeat by Japan in their World Cup opener.
The decisive goal came when Dalsuke Matsui's cross looped over Stephane Mbia and was met by Keisuke Honda, who fired in coolly at the back post.
"I'm upset we have lost the game but our attitude was wrong," said Le Guen.
"We were tense and nervous, especially in the first half. We did not show what we are capable of. We were not at our level and kept losing possession."
Cameroon, the highest ranked African team at the World Cup, were largely disappointing and with star striker Samuel Eto'o playing wide on the right, lacked cutting edge in front of goal.
Le Guen admitted his ploy to play the Inter Milan striker on the right flank did not work.
Honda puts Japan in front
"I chose the position for him and I take responsibility for that," he said. "You make the choice with what you have, and I wanted to do the best, but it didn't work."
Cameroon were expected to beat a Japanese team who arrived in South Africa without a win in five warm-up matches.
But the defensive lapse in the 37th minute proved costly, with Le Guen admitting: "My players were wrongly positioned."
Before the game, Le Guen had talked of matching Cameroon's achievement back in 1990, when the Lions reached the quarter-finals.
But they will need to beat Denmark in Pretoria on 19 June if they are keep alive their hopes of reaching the knock-out stages.
"We have to win against Denmark, we know that, we will try to recover well to do our best," said Le Guen.