Michael Phelps won the 17th Olympic gold medal of his career, and his 21st overall, when he produced a majestic performance in the 100m butterfly.
It was the final individual race of the 27-year-old's career which will end on Saturday night in the medley relay.
In Friday's race, the Baltimore swimmer was seventh at halfway, with nemesis Milorad Cavic first, before the American produced a customary storming final length to touch in 51.21secs.
Chad le Clos, who had relegated Phelps to second in the 200m butterfly earlier this week, was second.
The crowd rose as one to applaud the most successful Olympian of all time.
Earlier, Missy Franklin got the United States off to a good start in the pool as she won gold in the women's 200 metre backstroke in a world record time of 2:04:06 while Florent Manaudou won gold in the men's 50 metre freestyle.
