Olympics 2012: On the Scene at the Games in London (PHOTOS)
The 14-time Olympic champ, 27, competed in the qualifying round of the 200-meter butterfly earlier in the day, an event in which he is the two-time defending champion.
Surprisingly -- or maybe not, based on how his Games have unfolded so far -- Phelps finished third in his heat, and fifth overall, to land a spot in Monday night's semifinals.
If Phelps -- who on Sunday won his 17th Olympic medal, a Silver, in the 400-meter freestyle relay -- were to advance to the finals, he'd have a chance to tie former Soviet gymnast and 18-time medalist Larisa Latynina as the most decorated Olympian in history.
Of course, Phelps' appearance was just a small part of Monday's Games, which will soon see Team USA take on the world in the Men's gymnastics finals, at 11:30AM ET.
For more photos and results from Monday's events, click through our gallery, above.