Michael Phelps swept to a jaw-dropping 22nd Olympic gold on Thursday to become one of only three athletes to have won titles in the same individual event at four straight Games.
There was a stunning upset in the women's competition as Australian world record-holder Cate Campbell inexplicably flopped in the 100m freestyle, but there was no escaping Phelps as he romped to his fourth gold of the Rio Olympics.
The record-breaking American, whose place among the pantheon of sporting greats has long been cemented, stormed to victory in the 200 individual medley to match his London tally four years ago.
Competing in his fifth and final Olympics, Phelps also joined Al Oerter in the discus (1956-68) and Carl Lewis in the long jump (1984-96) as the only members of the exclusive four-in-a-row club.
Phelps utterly dominated a final which crackled with tension and brought the curtain down on his 12-year rivalry with fellow American Ryan Lochte.
After absorbing a strong start from local favourite Thiago Pereira, Phelps took charge on the backstroke leg before powering away to win in one minute, 54.66 seconds -- almost two seconds clear of Japan's Kosuke Hagino in second.
Hagino, who won the 400m medley last weekend, produced a quick finish to pinch silver in 1:56.61, with China's Wang Shun coming through for a surprise bronze in 1:57.05.
Lochte, a six-time Olympic champion, could only finish fifth, while Pereira's challenge faded badly as he trailed home seventh.
A little more than half-an-hour later, having collected his gold medal, Phelps returned to qualify fifth fastest for Friday's 100m butterfly final -- another event he won in Athens, Beijing and London.
Phelps could now finish with six gold medals, equalling his Athens tally and two shy of his epic haul in Beijing.
"I thought he did a really nice job," said his coach Bob Bowman. "He swam a great tactical race."
Campbell flop 
Campbell turned 0.12 inside her own world record before being caught by the two women on either side of her -- American Simone Manuel and Canadian schoolgirl Penny Oleksiak, who shared gold in 52.70.
Sweden's Sarah Sjostrom also crept by her to take bronze to complete her collection after winning gold in the 100m fly and silver in the 200m free. Campbell, incredibly, finished outside of the medals in fourth.
Manuel became the first American woman to win the 100m free since the 1984 Los Angeles Games, which was also a tie -- between compatriots Nancy Hogshead and Carrie Steinseifer.
"When I hit the wall I was just trying to catch my breath," said 16-year-old Oleksiak after becoming the first Canadian to win four medals in a single Games.
"I saw Simone's name and I thought I came second, then I saw 'Olympic record' by both names and I thought: 'That's crazy, no one ties at the Olympics.'"
Japan's Rie Kaneto blazed to gold in the women's 200m breaststroke, clocking 2:20.30 to deny two-time doping offender Yulia Efimova.
The Russian, whose participation has provoked open hostility from rival swimmers, was a distant second with China's Shi Jinglin third.
American Ryan Murphy completed a Rio double after bullying world champion Mitchell Larkin in the men's 200m backstroke, winning in 1:53.62 to underline his country's dominance of the event.
Australian Larkin, who failed to medal in the 100m, took silver and Russia's Evgeny Rylov the bronze.
"I remember looking up to all these guys, they were my idols," said Murphy, referring to Lenny Krayzelburg, Aaron Peirsol and Lochte, who were among the American winners at the past four Games.
"To think I could be that idol for some kid who is trying to do this is really cool," he added, as the Americans finished day six with 11 swimming golds, a whopping eight clear of Australia and Hungary.