Phelps made his country proud after dominating in the swimming pool and taking home eight gold medals, which set a new record for gold medals won in a single Olympic Games.
We knew that act would be tough to follow and that Phelps would piece together a slightly more modest performance during the 2012 Games in London, but perhaps the decline is worse than we thought.
Today Phelps was barely able to produce during the qualifier for the 400-meter individual medley.
Phelps won his heat with a finish time of four minutes, 13.33 seconds. It was enough to earn the eight spot in the final, but Laszlo Cseh of Hungary nearly spoiled Phelps' shot at winning a medal in the event after barely falling behind with a time of 4:13.40.
Phelps' decline may be more severe than we anticipated. But when will the champion swimmer's first failure occur?
Most likely, the final round of the 400-meter medley will mark his first display of decline in the 2012 Summer Games.
In the 400-meter qualifier, Kosuke Hagino of Japan led the way with an impressive 4:10.01 time, beating Phelps by more than three full seconds.
USA teammate Ryan Lochte finished with the third-best time at 4:12.25 and will also be competing with Phelps in the final round.
Will Michael Phelps upset his opponents in 400-meter medley final?
Phelps will be swimming with a disadvantage on the outside lane, so either Lochte or Hagino will have a realistic chance at spoiling his first shot at a gold medal.
Phelps' Gold Medal Outlook
Phelps won eight gold medals in Bejing but is only competing in seven events in London, so a repeat performance of equal or greater value was already impossible.
Phelps has a realistic shot at losing the 400-meter individual medley, but that's not to say he doesn't have a shot at winning in an upset, though it's looking unlikely at this point.
But if he does lose in the final, he will still be left with six more opportunities for the gold.
The final six events include: 200-meter freestyle, 200-meter butterfly, 4x200-meter freestyle, 200-meter individual medley, 100-meter fly, 4x100-meter medley.
The 400-meter medley was not considered Phelps' top event, so a loss will not be terribly disappointing, but it certainly makes it difficult for him to repeat greatness.
Also, Phelps will likely struggle in the 200-meter medley on Aug. 1, as he'll likely once again be facing Lochte.
Phelps' next event will be the 200-meter freestyle on July 29.