Michael Phelps of USA competes on the way to winning the gold medal of Men's 200m Individual Medley Final in Rio de Janeiro, Brazil, August 11, 2016. [Photo/Agencies]

"Right now I don't know how to wrap my head around that. I don't know what to say. It's been a hell of a career," Phelps told reporters, while acknowledging the strain on his 31-year-old body.

"That hurt a lot ... my body is in pain, my legs are hurting, I'm tired."

Despite winning four golds in London in 2012, Phelps has said he was dissatisfied with his preparation and results there and wants to bow out on his own terms.

"The biggest thing for me through the meet so far is I've been able to kind of finish how I wanted to. I've been able to come back and I've been able to accomplish things that I just dreamt of," he said.

Phelps' four consecutive golds in the same event are unique for a swimmer, and place him in the same Olympic pantheon as fellow-Americans Al Oerter in the discus and Carl Lewis in the long jump.