American Rebecca Soni beat her own world record as she won gold in the women's 200m breaststroke.
The 25-year-old clocked two minutes and 19.59 seconds to become the first swimmer to successfully defend an Olympic title at London 2012 and the first woman to break 2:20.00.
"We have never seen that before. The first woman under two minutes and 20 seconds. Second and third both went 2:20s too. This is just stunning, a wonderful swim. Because the rest of the field were so close, I wasn't sure Soni was swimming well, but she was. You have to give credit to Suzuki also. She was brilliant."
"I've been chasing that for years, that was for my old coach Tom who told me I was going to go that fast," she said.
Japan's Satomi Suzuki took silver while Russia's Iuliia Efimova finished third.
On breaking the historic mark, Soni added: "I kept it to myself as a secret goal, but that was for Tom. He believed in me more than any coach I've known, but as a young kid he was someone I looked up to and he kept pushing me."
Soni trailed South Africa's Suzaan van Biljon after 50m, but she broke free of the field down the second 50m and kept world-record pace throughout to beat Suzuki by 1.13 seconds and claim her second medal of London 2012 after her
silver in the women's 100m breaststroke.
It is also the USA's 11th gold in the Aquatics Centre.