Rio de Janeiro, Aug 10 (IANS): Olympic swimming legend Michael Phelps has said his victory in the 200 metre butterfly final along with his 4x200m freestyle relay gold at the Rio Olympics were the culmination 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, reports Xinhua.

Phelps also secured his third gold medal of the Olympics, 21st of his staggering career, in the 4x200m freestyle relay final on Tuesday.

Phelps joined Conor Dwyer, Ryan Lochte and Townley Haas to hit the wall in 7 minutes 0.66 seconds at the Olympic Aquatics Stadium.

Despite his overall dominance, the American had to settle for silver in the 200m butterfly category 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 on Tuesday.

"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," he said.

He did leave every ounce of energy in the pool, 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 the 200m butterfly.

"When I came back I said that I wanted the 200m fly," he said, adding that the realisation that this is 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," Phelps said.