Phelps dethroned defending 100m butterfly champion Ian Crocker with a storming last 20 metres to touch in 50.77 seconds, the second fastest time in history and the first time the American has gone below 51 seconds.

His last-gasp victory drew him level with Thorpe as the only swimmers to win six gold at the same championships and keeps him on course for an amazing eight titles with the 400m individual medley and the 4x100 medley relay to come.

"That's how I won the Olympic medal," he said of his narrow win.

"You have to nail the finish as best you can. I tried to get my hand on the wall and try to take it out in the first 50 as well."

His disappointed teammate Crocker took the silver in 50.82 with Albert Subirats Altes of Venezuela winning the bronze.

"It's never quite the race you want to (have). It was alright," said Crocker. "I always want to go faster. I always want to put my hand first on the wall."

The on-fire Australian women held off the United States to win the 4x100m medley relay gold medal, beating their own world record when anchor Libby Lenton touched in 3:55.74.

It was Lenton's fourth gold of the meet and Leisel Jones's third.

The US took silver and China won the bronze.

While Phelps added another chapter to his extraordinary story, Thorpe was reeling from a French newspaper report that claimed a sample of his last May showed abnormally high readings for testosterone and luteinizing hormone.

FINA said it had lodged an appeal with the Court of Arbitration for Sport on a doping test, seeking clarification of the adverse analysis, although it did not name the Australian swimmer who was poolside Saturday.

The threat to the reputation of Thorpe, who retired in November as one of the greatest swimmers in history, sparked a vigorous defence of his character from some of the world's leading coaches.

"He is a young man of unbelievably strong principles and integrity and I've known him for a long time so I will support Ian 1000 percent," said Swimming Australia executive director Glenn Tasker.

Back in the pool, Kate Ziegler upstaged French favourite Laure Manaudou to win the 800m freestyle gold in 8:18.52.

Manaudou took silver in 8:18.80 and Hayley Peirsol of the United States won the bronze (8:26.41.)

Ziegler's time was the second fastest in history after Janet Evans who still holds the world record (8:16.22) which she set in Tokyo in 1989.

"To be two seconds behind, I couldn't be happier," the American said of Evans' record. "I like to take my races out fast."

The American charge on the medals continued with Ben Wildman-Tobriner splashing his way to the 50m freestyle title in 21.88 with his teammate Cullen Jones second (21.94) and Sweden's Stefan Nystrand third (21.97.)

Another American Margarat Hoelzer swam the second fastest time in history to take the 200m backstroke gold medal in 2:07.16, with the 1991 record (2:06.62) of Hungary's Krisztina Egerszegi remaining intact.

Zimbabwe's Kirsty Coventry, the defending champion, took the silver and Reiko Nakamura of Japan won the bronze.

Sweden got its first gold of the meet when European champion Therese Alshammar sprinted harder than anyone else to win the women's 50m butterfly in 25.91 ahead of Australia's defending champion Danni Miatke (26.05).

Dutchwoman Inge Dekker (26.11) was third.

Earlier, Grant Hackett struck a psychological blow with a sub-15 minute heat win as he battles to hold on to his 1500-metre freestyle title.

Hackett, whose four-time world 1500m crown is on the line on Sunday, finished fifth fastest into the final behind Welshman David Davies (14:53.57).