Eighteen-year-old American Michael Phelps threatens to upstage the mighty Ian Thorpe when the main pool action of the world swimming championships unfolds over eight days starting on Sunday.

Thorpe has dominated the world of swimming since he became the youngest men's world champion in history as a 15-year-old Sydney schoolboy at the 1998 world championships in Perth.

Four times world male swimmer of the year in the influential Swimming World magazine, Thorpe has amassed a gleaming pile of gold medals -- a record-equalling eight from two world championships, three from the 2000 Sydney Olympics and heaps more from Commonwealth, Pan Pacific and world short-course championships.

Still only 20, his achievements have brought him wealth and fame. All eyes focus on Thorpe whenever he races because world records are always likely to tumble in demonstrations of unique swimming power.

But, for once, the Thorpedo may have to share the superlatives as a teenager from Baltimore enters the action in the Palau Sant Jordi pool amid anticipation that he, not Thorpe, will emerge as the most prolific gold medallist of the championships.

Since Thorpe set the last of his 17 individual world records a year ago, Phelps has racked up three, twice lowering the mark for the 400 metres individual medley and capping that on June 29, on the eve of his 18th birthday, by breaking the 200m individual medley standard - set in 1994 and the oldest men's record in the book.

GOLD QUALITY

Phelps, fifth in the 200m butterfly at the 2000 Olympics, set the first world record of his career in that event in March 2001 and lowered it in winning the 200m butterfly title at the world championships in Fukuoka in July.

He has also swum within 0.03 seconds of the 100m butterfly world record this year, so four Barcelona titles plus golden relay pickings in a strong U.S. squad look a distinct prospect.

Thorpe should achieve an unprecedented third successive world 400m freestyle title on the opening day and will be favourite to retain his 200m freestyle crown when he races against Pieter van den Hoogenband, the Dutchman who beat him in the Olympic final.

Van den Hoogenband, however, is likely to have the edge on Thorpe in the 100m freestyle, while Phelps and Italian Olympic champion Massimiliano Rosolino will make life difficult for the Australian in the 200m individual medley and the possibility of a repeat of Thorpe's three relay golds of 2001 is remote.

Phelps's morale could scarcely be higher. "I wanted to go into the world championships as the number one seed but I had no idea I'd go a 1:57," he said after his 1:57.94 eclipsed the 200m IM world mark.

Phelps is hugely versatile: he is also second in the world this year in the 200m backstroke and fifth in the 200m freestyle but will not swim those events in Barcelona.

BARCELONA SHOWDOWN

However, Australian former head coach Don Talbot says the American has yet to prove himself.

"It's a bit of one-upmanship between the Americans and the Australians. We've got Ian Thorpe and they're trying to say they've got someone even better," Talbot said after the latest world record from Phelps, which he conceded was "a great swim".

"In the major international meets Phelps has done nothing yet. Obviously something's going to happen at the worlds but I think the worlds will be a bit of a crossroads for him to see just what he can do when he gets up against the world's best."

Inge de Bruijn, the Dutch triple Olympic and triple world champion, and American Natalie Coughlin stand out among the women.

De Bruijn, returning after a year out of the spotlight, leads the way in the 50m freestyle and 50m butterfly, while Coughlin, the reigning 100m backstroke champion, is a formidable force in backstroke, butterfly and freestyle.

Grant Hackett looks set for double gold in the 800m and 1500 freestyle. Fellow Australian Thorpe, who beat him in the 800 in Fukuoka, is not defending that title, while Hackett has won every major 1500 he has swum since the 1998 world championships and is poised to make it three world titles in a row.