USA's Michael Phelps kisses his son, Boomer, held by his partner Nicole Johnson, as his mother, Deborah (second from right), looks on after he won the men's 200m butterfly final in Rio de Janeiro on Tuesday. Martin Bureau / AFP

By his own admission, Michael Phelps could not have scripted it better as he won back the title closest to his heart on an evening of pure gold for the most medaled Olympic athlete of all time.

The 200m butterfly was the first race he swam at an Olympics, at Sydney in 2000, winning the race. It was also the gold that got away - the one he wanted back after being beaten by a fingertip at the 2012 London Games by South African Chad Le Clos.

It hurt so much that Phelps made it his mission to exact revenge in Rio and win it one last time.

"Mission accomplished," he said on Tuesday after claiming the 20th Olympic gold of his career, a number that rose to 21 after he helped the US team win the 4x200m freestyle relay in the evening's finale.

Japan's Masato Sakai followed him to collect the silver in the butterfly, while Hungary's Tamas Kenderesi won bronze.

Le Clos, whose run-up to the Games was overshadowed by the devastating news that both of his parents had been diagnosed with cancer, finished fourth.

Phelps, who would have had four successive wins in the 200m butterfly but for that London race, has now amassed a record-extending 25 Olympic medals.

Some, however, carry far more significance than others.

"I really wanted that one back," said Phelps, who retired after 2012 but came back because he felt he needed to go out on his own terms.

"That event was kind of my bread and butter. That was the last time I'll ever swim it.

"Having that come to an end, it's crazy to think about.... I didn't say anything to anyone else, but there wasn't a shot in hell I was losing it."

Phelps said he had been prepared to leave "every ounce" in the pool, and strain every sinew, and did not know how close it had been until he was on the podium.

By then, none of that mattered.

"Just being able to see the number one next to my name again, one more time in the 200 fly, I couldn't have scripted it any better," he said.