Phelps put together his finest hour of the London Games, defeating a strong field that included his longtime rival Ryan Lochte to win the 200-meter individual relay -- a difficult race in which a competitor swims four different strokes over fourth lengths of the pool.
He won his 16th Olympic gold medal and 20th overall, making it that much more difficult for someone far in the future to break his career records. It was also his first individual gold of these Olympics and he became the first male swimmer in Olympic history to win the same event in three straight Games.
Soni, meanwhile, set a world record for the second straight night in the 200-meter breaststroke. She has won the race in consecutive Olympics.
On top of all that, Tyler Clary of the United States became the surprise winner of the 200-backstroke, an event in which Lochte had been the favorite.
Lochte finished third to Clary, however, and 30 minutes later came back to finish second to Phelps in the individual medley.
Although Lochte was clearly at a disadvantage having to swim the two races so close together, he appeared to have a stronger performance in the second one than he did the first.
The other final on the Thursday program saw Ranomi Kromowidjojo of the Netherlands capture the women's 100-meter freestyle. American Missy Franklin, who won a semifinal heat in the 200-meter backstroke earlier in the evening, got off to a poor start and failed to medal.
With five medals in swimming Thursday, the American contingent ran its total for the Olympics to 23. The United States has won 14 other medals at all the other venues.
Phelps got off to a sluggish start at the Olympics, failing to medal in the 400-meter individual medley after barely making the final and then losing gold by 0.05 of a second when he misjudged his finish in the 200-meter butterfly.
He looked like his old self Thursday in going out quickly and easily holding on to win the shorter of the individual medley races.
Phelps was timed in 1.54.27 -- a quarter of a second off his world record. At this late stage in his career, however, record times mean little to Phelps.
Soni, however, wanted another world mark and she got it by swimming the 200-meter breaststroke in 2:19.99. That was 0.01 of a second faster than the world record she turned in while winning her semifinal heat Wednesday.
Japan came up with two second-place finishes Thursday from Santomi Suzuki behind Soni in the breaststroke and Ryosuke Irie behind Clary in the backstroke.
Those two silver medals broke a streak of seven consecutive bronzes won by the Japanese in the pool. Japan has yet to win a swimming gold.