American teenager Missy Franklin won 100m backstroke gold ahead of Australia's Emily Seebohm, with Britain's Gemma Spofforth coming fifth.
Franklin clocked a time of 58.33 secs - 0.21 shy of Spofforth's world record - just 20 minutes after swimming in the 200m freestyle semi-final.
Seebohm, who led at the halfway mark, was fastest in the heats and semis but had to settle for silver.
Japan's Aya Terakawa put in an impressive performance to take bronze.
"Missy Franklin came in here as the new Michael Phelps. In America, that's a huge thing. She's a still young girl at 17. She stepped up when everyone thought Emily Seebohm would win. I couldn't see Franklin getting near her, but she played it well in the heats and semis and did the business in the final."
Franklin, 17, remains in contention for six medals at her first Olympics, after taking
bronze in the 4x100m relay,
She will also swim the 200m backstroke, 100m freestyle, 200m freestyle and 4x100m medley relay.
Franklin's performance to beat Seebohm, who had set the quickest time of the year in Sunday's heats, was even more impressive so soon after her swim in the 200m freestyle semi-final, where she qualified in eighth position. Seebohm, 20, can take solace from her silver, which comes after a 2011 badly affected by illness.
Shoreham-born Spofforth, the 2009 world champion, was down in sixth at the turn but displayed a trademark sprint finish to move up to fifth behind Anastasia Zueva.
It was a place lower than at Beijing in 2008 but was a heartening performance,
12 months after she considered quitting the sport.
"I've always said it's the journey and not the destination," she told BBC Sport. "I am disappointed but everything happens for a reason and I am just really enjoying my swimming right now.
"I have been rock bottom but it's great to be back enjoying life again and have my passion back for life and swimming. Training with Ryan [Lochte] and working with Olympians is what really inspires me and I work with children back at home and want to inspire them."
Watch a full replay of the final on BBC Sport's Interactive Video Player.