American freestyler Lindsay Benko has pulled out of the swimming world championships in Japan after breaking her kneecap in a bizarre training accident.

Benko, who won a relay gold at last year's Sydney Olympics, slipped on a pool ledge while training at a university in Fukuoka on Tuesday, hitting her left knee.

Benko waited more than a day before going to hospital in the hope that she may have only suffered bruising, but x-rays revealed she had fractured the kneecap.

A US team spokeswoman said Benko wanted to stay in Japan and watch the championships but would be flown home if surgery was required.

Devastated

"It's a terrible thing to happen to her, she's really devastated," the spokeswoman said.

"Hopefully she can stay here but we've sent the x-rays back to the States and they will decided whether she has to go home or not."

Benko won a gold medal in the American 4x200m freestyle relay team at last year's Olympics.

She was entered in three individual events, the 100m, 200m, 400m freestyle, as well as the relay for the world championships.