In an age where superlative sports performances automatically spark suspicion, Michael Phelps insisted on Friday that his record-setting swimming exploits are achieved fairly.

　　"Anyone can say whatever they want, I know, for me, I am clean," Phelps said after winning his sixth swimming gold medal of the Beijing Games.

　　Phelps is just two triumphs away from surpassing Mark Spitz's record of seven golds at one games, set in Munich in 1972.

　　His 12 career golds to date, including six from the Athens Games, are the most of any Olympian in any sport.

　　To underscore his supremacy, Phelps has captured all six of his golds in the Water Cube with world records.

　　Phelps's phenomenal effort has made him the story of the Games.

　　But the doping disgrace of one-time athletics golden girl Marion Jones, 100m world record-setter Justin Gatlin and Tour de France winner Floyd Landis, as well as the sordid tale of steroid use in Major League Baseball that emerged with the Mitchell Report, has made skeptics of sports fans in the United States and beyond.

　　Phelps is among the US Olympians taking part in a special US Anti-Doping Agency initiative called Project Believe, in which the competitors undergo extensive blood and urine tests beyond World Anti-Doping Agency (WADA) rules to create baseline body chemistry readings for hormones and other substances.

　　Altered levels of such substances often indicate doping, so it is hoped the extra tests can help restore credibility to sport.

　　Phelps said that with the programme he had done all he could do to prove he is clean.

　　"I did Project Believe, where I purposely wanted to do more tests to prove it," Phelps said. "People can question all they want, but I have the proof and the facts are the facts.