Phelps won the 400-meter Individual Medley race early Sunday at China's National Aquatics Center with a blistering time of 4:03.84. Phelps had set the previous best of 4:05.25 at the U.S. Olympic swimming trials in June.
Phelps was under world record pace at every turn. Even so, he said immediately after the event, "To be honest, I did not really feel that great. And going into the ready room, I started getting these, kind of like chills going through my body."
Placing second was Laszlo Cseh of Hungary (4:06.16) and Phelps' teammate, Ryan Lochte (4:08.09).
U.S. President George Bush and his father, former president George H. W. Bush, were in the stands amid fans waving American flags.
Phelps won six golds four years ago in Athens. In Beijing, he is seeking to break the record of seven gold medals in one Olympics, set by fellow American Mark Spitz in 1972 at the Munich Games.