Katie Ledecky started her opening-night program with a gold medal in the 200-meter freestyle at the Pan Pacific championships and finished it off by narrowly missing her own world record in the 800.
The pair of victories for 17-year-old Ledecky fulfilled expectations that a US woman would win two titles on Thursday.
Until Missy Franklin hurt her back in practice, though, it had been more likely to be the six-time winner at last year's world championships who doubled.
Ledecky, who entered the meet as the reigning world record-holder in the 400, 800 and 1,500 freestyle, said her first international title in the 200 gave her confidence in the later race.
She was a body-length ahead of world-record pace at one stage but faded slightly to finish in eight minutes, 11.35 seconds - she set the world mark of 8:11 in June. Lauren Boyle of New Zealand was second in 8:18.87 and Canadian Brittany MacLean was third.
She won the 200 free in 1:55.74, a meet record, ahead of Australia's Bronte Barratt and teammate Shannon Vreeland.
In the 100 backstroke, an event she won at the last Olympics and world titles, Franklin took bronze behind Australians Emily Seebohm and Belinda Hocking.
The Americans also collected gold in the women's 200 butterfly, with Cammile Adams winning in 2:06.61 from Natsumi Hoshi of Japan and US teammate Katie McLaughlin.
Australia's Thomas Fraser-Holmes won the men's 200 freestyle in 1:45.98, edging out Kosuke Hagino of Japan and another Australian, Cameron McEvoy.
Japanese men won the 100 backstroke and 200 butterfly.