With his 16th career Olympic gold medal, Phelps also became the first man to win swimming events in three straight Olympics.
American swimmer Rebecca Soni meanwhile beat the world record time she set just yesterday in the 200-meter breaststroke, becoming the first U.S. swimmer successfully to defend a gold medal.
Americans Missy Franklin and Elizabeth Beisel won the first and second semifinals in the women's 200-meter backstroke event, setting them up to compete in Friday's final.
More history was made when 22-year-old American Kayla Harrison defeated Britain's Gemma Gibbons for the first U.S. gold medal in judo. Harrison, who had been sexually abused for years by a former coach, thanked her "hero," coach Jimmy Pedro, who won bronze medals in the 1996 and 2004 Olympics.
In rowing on Dorney Lake, where Britain won its first gold medal Wednesday in the coxless pairs event, an American crew defended the U.S. gold medal in the 2,000-meter women's eight final. They have been undefeated since 2006.
In women's gymnastics, 16-year-old Gabby Douglas took the all-around title, becoming the first African American Olympic all-around champion and continuing an American winning streak in the event dating back to 2004.
Fellow gymnast Aly Raisman lost a tiebreaker to narrowly miss the bronze.
Dressage, a sport not typically in the media spotlight, drew reporters in droves to cover the performance of Rafalca, the 15-year-old mare co-owned by Ann Romney, wife of Republican presidential candidate Mitt Romney. Though Jan Ebeling, the horse's rider, beamed brightly as he strode off after the performance, Rafalca placed a disappointing 13th.
In tennis, Serena Williams handily defeated the Dane Caroline Wozniaki in two sets, 6-0, 6-3, advancing to the semifinals, where she will face Victoria Azarenka of Belarus.
Williams will compete in the women's doubles event with her sister Venus.