China's Sun Yang proved he has few peers in the distance freestyle events, while US star Missy Franklin claimed the first of what could be eight golds at the swimming world championships.
Sun, 21, won the men's 400-metres freestyle gold to complete the set of major titles in the event, 12 months after winning the Olympic crown.
Just as he did in London last year, the Chinese swimmer dominated early to add the 400m to the 800m and 1,500m freestyle golds that he won two years ago in Shanghai.
"The 400m is a very important event and I completed my goal," said Sun, who has recently moved to Hong Kong after a falling-out with his previous coach concerning his girlfriend, a flight attendant.
"I overcame many problems and proved myself. I hope to have an even better performance in my other events."
He will start the defence of his 800m freestyle title in today's heats.
Sun clocked three minutes, 41.59 seconds, finishing more than three seconds ahead of the field, to cap a bright start for China on the first of the eight-day long championships.
"I had a pretty good time for me because I don't have a major challenger here," said Sun, who won two golds, a silver and a bronze at last year's London Games. "If I had had one, I would have gone a lot faster."
Park Tae-hwan, of South Korea, who won the 400m at the Beijing Games and finished second to Sun in London, is not competing, and the field also missed world record holder Paul Biedermann of Germany.
Biedermann swam 3:40.07 at the 2009 world titles at the height of the rubberised-suit era.
The USA claimed two world titles as Katie Ledecky, 16, just missed the women's 400m freestyle world record on her way to winning gold, finishing more than two and a half seconds ahead of her rivals.
The teenager, who was a shock 800m freestyle winner at the Olympics, just missed the four-year-old world record.
"I really wasn't expecting to go that fast, that wasn't my focus. I just wanted to compete with these girls and get the gold medal," said Ledecky, who will also race in the 800m and 1,500m.
The Americans had more success as Franklin swam the first leg to lead them to the women's 100m freestyle relay gold.
Franklin, who won four golds at London, is bidding to become the first swimmer - male or female - to win eight gold medals at a world championships, a feat not even 18-time Olympic gold medallist Michael Phelps achieved.
The US teenager will swim in the 50m, 100m and 200m backstroke events, the 100m and 200m freestyle, with two more relays to come.
The USA won on Sunday by just 0.12 of a second from the Australians.
It was Megan Romano, swimming the anchor leg, who managed to hold off Alicia Coutts, leaving the Australian in floods of tears.
The USA nearly made it three titles in the men's 100m freestyle relay final, but France's Jeremy Stravius hit the wall just 0.24 seconds faster than James Feigen.