Michael Phelps said that his victory in the 200m butterfly final at the Rio Olympics on Tuesday was the accomplishment of a four-year mission.

Phelps won the event at the 2004 Athens Games and again in Beijing four years later. He also set the 200m butterfly world record at the 2009 world championships in Rome, a mark that still stands today.

Despite his dominance, the American had to settle for silver at the London 2012 Games, where he was pipped by his South African rival Chad le Clos.

"This is a race that I really wanted," Phelps said of his victory at Rio's Olympic Aquatics Stadium.

"I really wanted that one back. Tonight I came in the pool with a mission and the mission was accomplished. There wasn't a chance in hell I would lose that tonight and if I did every ounce of energy would have been left in the pool."

Leave every ounce of energy in the pool he did, staving off a storming finish from Japan's Masato Sakai to win by just four hundredths of a second.

"The last 10 meters wasn't fun. I felt like I was standing still," Phelps said.

The 31-year-old admitted that one of the main factors in his decision to return to swimming after his 2012 retirement was to seek redemption in he 200m butterfly.

"When I came back I said that I wanted the 200m fly," he said, adding that the realization that these are his last Olympics was starting to sink in.

"Just to see my name beside the 200m fly again... I couldn't have scripted it better. The event was my bread and butter. Having that come to an end is weird. It's crazy to think about."

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