Michael Phelps' hopes of winning eight Olympic gold medals were dashed after South Africa beat the USA into third in the men's 4x100m freestyle relay.
South Africa's squad celebrate their amazing win
South Africa's Ryk Neethling, Roland Schoeman, Darian Townsend and Lyndon Ferns swam superbly to set a world record of three minutes 13.17 seconds.
The Netherlands took silver in 3:14.36 while the USA's time was 3:14.62.
Phelps, who has already won one gold, could still emulate Mark Spitz's 1972 record of seven gold medals.
The South African quartet's astonishing time beat the previous mark by more than half a second.
"We wanted to do better, but this is what we had," said Phelps, who swam the second leg of the relay.
He refused to blame his team-mates - Ian Crocker, Neil Walker and Jason Lezak - for the disappointing showing.
"These are the four fastest guys we had," he said.
"This is only the second time in history we got beat (in this event)."
Earlier, Britain's James Gibson and Darren Mew both missed out on a medal in the men's 200m breaststroke final as Japan's Kosuke Kitajima took victory.
Kitajima clocked 1:00.08 seconds to edge out the USA's Brendan Hansen into the silver medal position, while France's Hugues Duboscq took bronze.
Gibson and Mew both appeared to be making good progress but faded as the finishing line drew near.
The British pair eventually finished sixth and seventh respectively.
"I'm just gutted. I slipped a bit in the first 50m, but that was terrible," a devastated Mew told BBC Sport.
Gibson also admitted that the pair had not performed as they would have hoped.
"It's not good enough to be in with a shout at 50m - you have to be there at the end," said Gibson.
"We've both put so much work in but we got the result we deserved."
Ian Thorpe and Phelps set up an enticing prospect as they both qualified for the men's 200m freestyle final.
Going head-to-head in the second semi-final, Thorpe finished in 1:46.65 seconds for victory with Phelps just behind in a tactical race, in which both appeared to be keeping plenty in reserve.
Phelps and Thorpe size each up before their freestyle semi-final
The high-quality final will also feature Australia's Grant Hackett and defending champion Pieter van der Hoogenband, who set the fastest qualifying time at 1:46.00.
And Britain's Simon Burnett gave the best performance of his career to qualify in third place behind Hoogenband and American Klete Keller.
Burnett broke the British record with his time of 1:47.95 to finish third.
"All my coaches said I was capable of making an Olympic final and that I could break the British record," said Burnett.
"I'm just glad I could prove them right."
In the semi-finals of the men's 100m backstroke, Aaron Peirsol of the US put down a marker in winning the first race in the fastest time of 54.34.
Austria's Markus Rogan also looked good as he finished with a time of 54.42, with Japan's Tomomi Morita winning the second semi from Lenny Krayzelburg.