Chinese prodigy Ye Shiwen won her second gold of the Games, setting an Olympic record in the 200m individual medley, after stunning swimming pundits with her victory and world record in the 400 medley on Saturday.
But the night belonged to Phelps, who swam the anchor leg of a relay that the Americans dominated from start to finish after Ryan Lochte handed them a commanding lead.
It was a historic moment in the 116-year annals of the modern Olympic Games, and an emotional one for Phelps, still a powerful force but no longer the commanding figure who won an unprecedented eight golds at the Beijing Games in 2008.
His team mates flung their arms around him. “I thank those guys for helping me get to this moment,” said the 27-year-old from Baltimore, who had 16 medals before the start of the Games.
Having picked up a silver in Sunday’s 4×100 freestyle relay, he drew level with Soviet gymnast Larisa Latynina’s tally of 18 when he won silver in his first outing of Tuesday night, in the 200 metres butterfly.
The American led going into the last few metres of his favourite race, but was tiring rapidly and had to settle for second when South Africa’s Chad Le Clos ploughed through to snatch it on the final touch.
Watching at poolside was Latynina, 77, who has held the record for decades with her haul from the Games at Melbourne (1956), Rome (1960) and Tokyo (1964).
Latynina told Reuters earlier this month she had no doubt Phelps would overtake her in London, adding: “I can only wish him well”.
The entire audience in the 17,500-capacity Aquatics Centre stood to applaud Phelps’ cumulative achievement in winning 15 gold medals, two silvers and two bronze. Latynina won nine, five and four respectively.
In other action on Day Four, Germany won its first two golds, in equestrian eventing, and France its fourth, in canoe slalom, but host nation Britain was still seeking its first.
China tops the overall medals table with 13 golds, followed by the United States with nine. Each has 23 medals in all.
The US women’s team stormed to the gymnastics gold – the country’s first since 1996 – with dazzling performances from Jordyn Wieber and Gabby Douglas.
It was compensation for Wieber’s disappointment at missing out on a spot in the all-around individual final, and for the U.S. men’s slump to fifth place in their team event on Monday.
In the pool, 16-year-old Ye completed a medley double when she held off a late challenge from Alicia Coutts of Australia.
The teenager, who swam her last length on Saturday faster than Lochte did in winning the corresponding men’s event, has been forced to fend off insinuations of cheating.
American John Leonard, executive director of the World Swimming Coaches Association but not on the US coaching staff in London, told Britain’s Guardian newspaper: “Every time we see something … ‘unbelievable’, history shows us that it turns out later on there was doping involved.”
The US Olympic Committee distanced itself from his comments, saying he was not associated in any way with the country’s swimming or Olympic team.
Chinese officials hit back. “Ye Shiwen has been seen as a genius since she was young, and her performance vindicates that,” Xu Qi, head of the Chinese swimming team, told the news agency Xinhua.
“Don’t use your own suspicions to knock down others. This shows lack of respect for athletes and for Chinese swimming.”