Michael Phelps left as he began: Winning.。

The most decorated athlete in Olympic history won gold in Rio de Janeiro in what he insists will be his last-ever competitive swimming event. 。

Taking out the 400 medley relay Saturday, Phelps and his teammates, Ryan Murphy, Cody Miller and Nathan Adrian, dominated from the starting gun.。

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An emotional Phelps, 31, took a moment to turn and face an adoring crowd after the race. His mother, Debbie, and fiancé, Nicole Johnson, watching in the stands with tears in their eyes. Phelps' son Boomer, quickly becoming a social media star himself, was also there to witness his father's triumph.。

Nevertheless, Phelps remained characteristically understated as he took a lap of honor with his team around the pool, U.S. flag in tow. 。

Twitter lit up with tributes, not least from the White House.。

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It wasn't an entirely smooth run at the 2016 Olympics. Phelps was held off from winning gold in his last Olympic individual race Friday night by 21-year-old Joseph Schooling from Singapore, who beat his idol in the 100-meter butterfly.。

Phelps has maintained he won't be coming out of retirement again for Tokyo in 2020. "I'm not going four more years and I'm standing by that," he said.。

"I've been able to do everything I've ever put my mind to in the sport and after 24 years in the sport, I'm happy with how things finished."。

The swimmer debuted at the 2000 Olympics in Sydney at the young age of 15. He went home empty handed, but kicked off a 16-year campaign that would leave him with 23 gold medals and 28 medals overall.。

In a Facebook video posted before the event, Phelps insisted Saturday's race was the last of his career. "Go for 30 medals in Tokyo? I don't think so boys and girls. This is over. This is it," he said.。

Plenty of Americans and swim fans around the world will be wishing it wasn't so. 。

Plenty of Americans and swim fans around the world will be wishing it wasn't so.。

The Associated Press contributed reporting.。

BONUS: Michael Phelps' Olympic vault。TopicsOlympics。