Michael Phelps captured his third gold medal on a controversial and dramatic night at the world championships on Thursday.
The American won the 200 metres individual medley title an hour after he flopped in the 100m freestyle final surprisingly won by Italian Filippo Magnini.
Otylia Jedrzejczak of Poland then broke her own world record to win the women's 200m butterfly final after a thrilling duel with Australia's Jessicah Schipper.
However, television replays suggested Jedrzejczak should have been disqualified for an illegal touch, although she was allowed to keep her gold medal.
Australians officials decided not to lodge a protest but called on swimming's world governing body to review their existing regulations on the use of video evidence.
Australia did grab one gold on Thursday when Giaan Rooney won the women's 50m backstroke final but the United States kept their lead on the medals table through Phelps and the women's 4x200m freestyle relay team.
Phelps joined Ian Thorpe and Grant Hackett as the only swimmers to win six individual world titles with his victory in the medley, but his failure in the 100m ended his chances of winning at least seven and equalling Mark Spitz's tally from the 1972 Munich Olympics.
Phelps had needed to win Thursday's blue-riband sprint to keep alive his dream of breaking Thorpe's record of six gold medals at a single world championship but he could only finish seventh as Magnini pinched the title with a perfectly-timed swim.
The Italian stormed home on the last lap after being fourth at the turn to flash pass the South African duo of Roland Schoeman and Ryk Neethling.
Magnini's winning time of 48.12 seconds made him the second fastest man in history behind world record holder Pieter van den Hoogenband.
"For me, it was a dream to be at the top," Magnini said. "I thought it would be third place because the two (South African) athletes are very strong."
Schoeman had led at the halfway mark almost three quarters of a second under van den Hoogenband's world record pace but weakened to finish second in 48.28, while Neethling also faded to take third in 48.34.
Jedrzejczak overtook Schipper in the final stroke to win gold in 2:05.61, wiping 0.17 off the previous record she set at the 2002 European championships in Berlin.
Schipper, who won the 100m butterfly title earlier this week, also went under the old mark, finishing in 2:05.65 and was reluctant to weigh into the unfolding drama over Jedrzejczak's final touch.
"I'm just gonna drop it," she said. "It was a close race but I am very happy with silver. I just wanted to do a personal best so to do that by two and a half seconds is fantastic."
Despite having to back up just an hour after the 100m, Phelps was determined not to dwell on his defeat as he climbed back onto the blocks for the medley final.
There was a hint of a boilover when Phelps trailed for the first 100m as Hungarian Laszlo Cseh went through the butterfly and backstroke legs under world record pace, but the American was in front when they switched from breaststroke to freestyle for the last lap.
Phelps pulled away to win in 1:56.68, three quarters of a second outside his own world record, with Cseh second in 1:57.61 and American Ryan Lochte third in 1:57.79.
"It was very tough doubling up but I've got no regrets," Phelps said. "I was just destroyed (in the 100m) but I just wanted to put that race behind me."
Rooney won the backstroke sprint in 28.63 to add to the 200m freestyle gold she won at the 2001 world championships in Fukuoka, Japan.
Gao Chang of China finished second in 28.69 while Antje Buschschulte was third in 28.72. Her German team mate Janine Pietsch finished sixth, just two months after she set the world record at 28.19.
Natalie Coughlin, Katie Hoff, Whitney Myers and Kaitlin Sandeno combined to win the 4x200m relay in 7:53.70 after a titanic battle with Australia.
Libby Lenton had given the Australians a big lead when she completed the first leg in 1:57.06, a second and half faster than Solenne Figues's winning time in the individual 200m final.
The Australians were still in front after 700m but Linda MacKenzie could not hold off Sandeno in the last 100m and had to settle for second in 7:54.06 with China third in 7:57.29.
Australia's Olympic champion and world record holder Jodie Henry set the fastest qualifying time for Friday's women's 100m freestyle final.
Her compatriot Leisel Jones was quickest in the 200m breaststroke while Americans Aaron Peirsol and Brendan Hansen led the way in the 200m backstroke and breaststroke respectively.
