For more than a decade, the great American had vanquished all comers in the 200 meters butterfly on both the world and Olympic stage, but then along came Chad le Clos.
"When I touched at 150 (meters) I think I was 1.5 meters behind him. When I turned I actually looked at him underwater and I thought I was him -- I know it sounds absolutely crazy, but I saw myself as him going past someone else."
"I remembered how he used to come off the last turn and, you know, smoke everyone ... I thought, he's done this for so many years and I remembered that when I was swimming. I don't know what it was but it was magical."
Making a splash
The mind-altering moment quickly morphed into a life-changing experience for the then 20-year-old who edged out Phelps in the final stroke.
"It was such a huge moment for South Africa. Obviously, to represent your country is a huge honor but to beat Michael Phelps at the Olympics was amazing," he says.
"As a young kid everyone wants to be like their heroes but you don't actually think one day that you're going to beat them."
The remarkable victory was memorable not just for le Clos' reactions -- which went from joy to disbelief immediately after the race -- but for those of his father Bert, whose elated response and subsequent TV interview
have become an enduring part of the swimmer's story.
The excitable tribute to his "down-to-earth, beautiful boy," which included repeated mentions of the word "unbelievable," touched the hearts of viewers and turned le Clos senior into something of a household name himself.
"I never knew my father was such a celebrity until like five, six days after the Games -- I was so wrapped up in (my own) bubble," le Clos explains.
"We were walking down the street and everyone was taking photos of me, but I couldn't find my dad, and I saw him and there were a line of people taking photos of him and I was like: 'Dad! You are stealing my thunder here. C'mon!' It was incredible.
"Everywhere he goes he's that famous dad and that's amazing because he really is. Nothing was put on for the cameras -- that's how he is back home. To my other brothers and sister he's a great dad."
Success in the pool has been rich reward for Bert, who steered the Chad towards swimming when a football career looked more likely.
"My parents were hugely influential in helping me make decisions throughout my life, especially my sporting career," he says.
"When I was young I played football until I was about 13 or 14 years old -- I played for the state team (Natal). I had to make a decision and my dad realized that I was a better swimmer, even though my family had a football background.
"Ninety-nine percent of fathers would have told their son to play football ... I still thank him every day for helping me make that decision."
There is now another le Clos in the pool -- teenager Jordan is hoping to follow in his brother's large footsteps, having already competed at South Africa's national championships.
"You know, 2020 is very important to me," says Chad. "Hopefully my brother can make the Olympics then and swim in the same relay team."
Originally a breaststroker, le Clos switched to butterfly after he tore both his abductor muscles (in the groin) in 2008 when he was 16 years old.
"Butterfly movement is really different. It's unlike freestyle or other strokes where it's very, very technical. I compare it to dancing -- I'm a terrible dancer! -- it's about getting your hips right, your kick is very important. I just really enjoy it. It's a fast stroke, so I guess the injury was a blessing in disguise for me."
While swimming's toughest stroke remains his favored discipline, le Clos has also struck gold in the individual 200m freestyle -- at the short-course world championships in Doha last year -- and the 400m medley (butterfly, backstroke, breaststroke and freestyle) at the 2010 Commonwealth Games in Delhi.
But it is butterfly where he has dominated in recent years. After pipping Phelps in the 200m in London, le Clos had to settle for silver behind the American in the 100m, but he hasn't missed a stroke since.
A 100m and 200m double at the 2013 World Championships in Barcelona was followed by gold at the Commonwealth Games in Glasgow, Scotland last August.
He capped a brilliant 2014 in December with three more titles (50m, 100m and 200m) at the short-course worlds in Doha before being crowned FINA World Male Swimmer of the Year -- an award he is incredibly proud of.
"The Olympic gold will always be there and it will be the most outstanding moment of my life, but to win the 2014 Swimmer of the Year award was up there. Not many people have won the Ballon d'Or of swimming, so it was a very special moment for me."
For le Clos, who turns 23 in April, the best is almost certainly yet to come with another Olympics fast approaching.
He concedes that it will be "very, very difficult" to get anywhere near Phelps' record haul of 18 Olympic gold medals (and 22 overall), but he may get the chance to race him one more time.
Phelps, who announced his retirement following the 2012 London Games, made a shock return to the pool last year although things haven't exactly gone to plan -- the "Baltimore Bullet" is serving a six-month ban imposed by USA Swimming after he was arrested for drink-driving last September
The ban has put his planned appearance at this year's FINA World Championships
in doubt, but Phelps is still hopeful of qualifying for his fifth Olympics in 2016.
"I really, really hope that he and his team decide to swim in Rio, I really believe it will be great," le Clos told Reuters last year
"It's added motivation for me ... with Michael back, it's really sparked my fire, so to speak. I don't think he will be worse in Rio, I think he'll be back where he wants to be. He's a champion in all respects but I believe I can beat him again."
If he can, then le Clos will have taken a giant step towards his ultimate aim of swimming and sporting immortality.
"I want to cement myself in the sport as one of the greats. In swimming terms, I want people to remember Chad le Clos -- the guy that not only beat Michael Phelps, but who is the best fast swimmer of all time," he says.
"In South Africa we have a rich history of great champions -- rugby players, cricket players, a lot of great golfers -- so I'm among really tough competition, but I believe that after 2016 and 2020 I can hopefully be the greatest."