Hungary's Katinka Hosszu produced one of the great Olympic swimming performances to shatter a four-year-old world record by over two seconds in the 400 metres individual medley final here this evening.
It came on a night otherwise notable for two Australian gold medals, meaning the Oceanic nation have already improved upon their sole victory in the pool at London 2012.
Mack Horton claimed 400m freestyle spoils before the women's 4x100m freestyle relay quartet took the title - and the world record to boot.
Japan's Kosuke Hagino also claimed gold and the first swimming title of the Games in the men's 400m individual medley.
But it was Hosszu, the "Iron Lady" of Hungarian swimming who failed to win any medals at London 2012, who responded so brilliantly to top the bill.
After nearly breaking Ye Shiwen of China's record mark in the heats, the 27-year-old set off at a blistering pace on the final.
At one point, she appeared to be four seconds up on the world record before slowing, only slightly, to clock 4min 26.36sec.
“It’s just crazy that I’ve been able to swim two seconds faster than anyone ever, ever did,” she said afterwards.
“It’s just crazy.”
Maya Dirado took United States silver in 4:31.15, while Mireia Belmonte out-touched Britain's Hannah Miley to take the bronze, in 4:32.39.
Defending champion Ye finished 27th in the heats.
Hosszu could yet become the most decorated athlete of these Games.
The five-time world champion is heavy favourite for the 200m individual medley and could also challenge in the 200m butterfly, 100m backstroke and 200 backstroke.
Horton produced a superb second half of the race to claim a surprise 400m freestyle victory over defending champion Sun Yang of China in 3:41.55.
Both men came charging through the field, but Sun - who served a three month doping ban in 2014 - did not quite have enough and could only touch second in 3:41.68.
Italy's Gabriele Detti took a distant bronze in 3:43.49.