International Olympic Committee president Thomas Bach hailed the two superstars as "icons". But he will leave the 31st Olympic Games wondering how to fill their places.
There is noone in sight with the sporting power nor the charisma to take the place of Phelps and Bolt -- who have 32 gold medals between them -- in the swimming pool or on the running track.
"The Greatest" maybe just Muhammad Ali-style hyperbole that Bolt likes to throw about but his nine golds over three Olympics cannot be countered.
"We have seen athletes who were icons even before they arrived here, they have strengthened their position as icons, like Michael Phelps and Usain Bolt," Bach said Saturday.
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From Beijing in 2008 through London 2012 and, finally, in Rio, Bolt and Phelps captivated die-hard fans of their sports and casual spectators attracted like moths to the Olympic flame.
- Perfect pair -
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Both presaged their mature exploits with precocious Olympics appearances, Phelps as a 15-year-old contesting the 200m butterfly in 2000, and Bolt, at 17, finishing fifth in his heat in the 200m in 2004.
In Beijing he cemented his place among Games greats with a perfect eight golds in eight events at the Water Cube while Bolt electrified the Birds' Nest stadium with his 100m, 200m and 4x100m sprint triumphs.
Although a burned-out Phelps flirted with retirement
after London, it was fitting that his decision to return for one last, fifth, campaign, saw him bow out at the same time as Bolt. The Jamaican's unprecedented third sweep of the 100m, 200m and 4x100m means debate will rage loud and long as to which can claim the status of "greatest Olympian".
Bolt, meanwhile, has dominated in the tests of speed that are the quintessential sporting contests, maintaining his supremacy over an unprecedented span of years.
"I've proven to the world I'm the greatest," declared the ebulliant Jamaican, who preens and poses with the same unabashed ardor with which he runs.
"It's a massive gap, but it's not a gap that is insuperable," International Association of Athletics Federations president Sebastian Coe said of Bolt's departure.
He compared it to questions asked with Ali's withdrawal in the 1970s. "Well, Floyd Mayweather, Marvin Hagler, Manny Pacquiao and Sugar Ray Leonard come along."
But the American revelled in his Rio triumphs, and showed a softer side with infant son Boomer included in his victory celebrations.
"As a kid I set out to do someting that nobody had ever done before," Phelps said. "I can look back at my career and say: 'I did it.'"
Bolt, now headed into a year-long victory lap that will culminate with the World Championships in London, can say the same.
Bolt has a wealth of riches to carry out his wishes. The Olympic hopes may not be answered by the time Tokyo 2020 starts.