To Michael Phelps, getting back in the pool at the 2016 Olympics in an attempt to add to his unprecedented medal haul does not represent him reneging on a vow to retire he made four years ago. He just happens to be in a completely different and much happier place than he was then.

The 18-time Olympic champion feels he might just be in the best shape of his life at the age of 30, but his mindset is the biggest difference heading towards the Rio Games.

For the first time in his professional life, Phelps has found renewed energy in letting go and rebooting, after dedicating so many years to the relentless pursuit of perfection.

"In 2012, I wanted nothing to do with the sport anymore," he said at a news conference during the United States Olympic Committee media summit. "I was ready to move on, ready to retire.

"I don't really know what I wanted to do at that point, where I wanted to go, but I was just spent, I was over it, I didn't want to train anymore."

But about a year and a half later - and 30 pounds heavier - he got back in the pool and found himself reinvigorated. He had regrets about the way he had trained heading into the London Olympics and was disappointed in his approach, even though it produced six more medals.

A nagging feeling had him worried that he might look back 20 years down the road and feel like he let himself down by walking off the world stage on what he considered a down note.

That spurred a return to competitive swimming with a twist: a more laid-back approach than he had ever employed before.

"Just living a sort of freer, happier life now is something that is a huge change," he said. "I don't feel like I'm carrying weights around anymore. I feel like whatever I've been holding inside me, I've just been able to get it out and kind of start fresh. It's a pretty incredible feeling."

Phelps made it clear that changes in his life away from the pool have been a driving force in his new approach. Now settled down with his fiancee Nicole Johnson and eagerly anticipating the birth of their son later this spring, he also has not had a drink of alcohol in about 18 months, since a DUI arrest in 2014 that led to a six-month suspension by USA Swimming. That change has Phelps feeling "completely clear-headed" and ready to attack one last challenge.

And yes, this summer really will be the last time the world sees Phelps competing on the biggest stage.

"I think I'm in a different place now and when I say this is my last one, this is my last one," he said. "I think when this summer is over, as long as I'm able to look back to say I did everything I could to get ready, it doesn't matter what happens in anything else.

"If I don't win a single medal and I do everything I can to prepare myself … that's all I'm asking for. That's all. Because that's all I could have done."