Michael Phelps, who has won five gold medals in the Olympic pool, said on Friday he was pulling out of Saturday's 4x100 metre medley relay final to give team mate Ian Crocker the chance to swim.
Having swum in the relay heats, Phelps would still get a medal and so stay on course for an Olympic swimming record eight in one Games if the American team finishes in the top three on Saturday.
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Kirsty Coventry wins 200m backstroke
Hall retains 50 metre freestyle
Phelps, who beat Crocker in a desperately close 100 metre butterfly final on Friday, said he wanted his team mate to have the spot because he had not been feeling well when he swam in the U.S. 4x100 freestyle relay squad, who were beaten into third place in the final last Sunday.
"I wanted to give him another chance," Phelps said.
Phelps became the first man since Mark Spitz to win four individual golds in a single Games when he won Friday's race. The victory earned him the right to swim in the medley relay final and win a sixth gold and he might still get it.
"Michael is already assured whatever medal the relay wins tomorrow night," said Phelps's coach Bob Bowman.
U.S. head coach Eddie Reese made the surprise announcement at a news conference and said the decision had been Phelps's.
"I never talked to him about it," Reese said. "Since the race was so close at 4/100s (0.04 seconds) and Ian's relay take-off is so much better ... Ian is going to swim fly in the medley relay tomorrow evening.
"It's a helluva gesture, it's correct, but it's a helluva gesture," Reese added.
Phelps said giving up his place was tough but the best decision for the team.
"Ian is one of the greatest relay swimmers in the U.S. team and he wasn't feeling too well in the freestyle relay," he said. "But I am willing to give him a chance to step up tomorrow night and hopefully win that relay.
TEAM WORK
"We came into this meet as a team and we're going to leave it as a team.
"I'm willing to give him another chance. He wasn't feeling well earlier in the meet but now he's started to feel better and better every day, so I'm willing to give a team mate another chance. We're going to put the four fastest guys up there."
Phelps has won gold in the 200 and 400 individual medley, 100 and 200 butterfly and 4x200 freestyle relay. He has also collected bronzes in the 200 freestyle and 4x100 freestyle relay.
The 100m butterfly brought him his seventh Olympic medal, matching the total achieved by Spitz in 1972 and a third American Matt Biondi in 1988.
His Olympics over, Phelps is now looking forward to indulging in some fast food -- hamburgers, fries and apple pie.
"It's a good thing to be able to sit back and relax a little but when tomorrow's final comes I will be in the stands and cheering as hard as I can for the U.S. team," he said.
Kirsty Coventry wins 200m backstroke
Kirsty Coventry of Zimbabwe won the gold medal for women's 200 metres backstroke.
The 20-year-old finished first in two minutes 09.19 seconds, outside the world record of 2:06.62 set by Hungarian Krisztina Egerszegi at the 1991 European championships, also in Athens.
Stanislava Komarova of Russia finished second in 2:09.72 to win the silver medal while the bronze was shared by Reiko Nakamura of Japan and Antje Buschschulte of Germany who both swam 2:09.88.
Coventry had already won a silver and bronze at the Athens Games.
Hall retains 50 metre freestyle
American Gary Hall retained his Olympic men's 50 metre freestyle title with victory in the sprint final.
Hall won gold in a time of 21.93, outside the world record of 21.64 which belongs to Russian Alexander Popov, who failed to get through the heats.
Duje Draganja of Croatia was second in 21.94 for silver, while South African Roland Schoeman took bronze in 22.02.
Hall shared gold with compatriot Anthony Ervin at the Sydney Olympics four years ago after the pair finished in a dead heat.