Olympic organisers on Wednesday defended the switching of a seven-year-old singer with a more photogenic double during the opening ceremony, comparing it with an athlete getting dropped for a big game.

Nine-year-old Lin Miaoke became a celebrity in China after she "sang" a patriotic ballad in front of 91 000 people and a television audience of millions during Friday's opening spectacular.

But organisers later admitted the real voice was that of chubby Yang Peiyi, who was deemed not attractive enough to go on stage, a decision that the International Olympic Committee's executive director Gilbert Felli defended.

"You have to be sure that the performers and the song, it is at the highest level," he told reporters.

"It is casting.... it is a technical decision by the producers. (You have to) put that in the context of the opening ceremony and the complexity of the 15 000 performers."

Felli added that the decision was like a player being part of a team during the qualification for the Olympics, but being replaced for the main event.

"That is what it is in sport, in life," he said, after he was asked how Yang's parents would explain the decision to her.

Wang Wei, vice president of the Beijing Olympic organising committee also said he saw no problem with the secret move.

"I think it is a decision of the group of the directors, together... they are to achieve the most theatrical effect for the benefit of the whole performance, the whole opening ceremony," Wang told reporters.

"I do not see there is anything wrong with it," he said.