RIO DE JANEIRO – Cate Campbell took nothing for granted Saturday. Though she was expected to lead the Australian women to an Olympic gold medal in the 4x100-meter freestyle relay, she quieted those who assumed after the preliminaries that the Aussies had it in the bag.

"You can't read into anything before an Olympic final,'' Campbell said. "That's why they call it the Olympic Games, because something special always ­happens.''

The result might not have been unexpected, but Campbell guaranteed it would be special. She anchored Australia to its second consecutive Olympic gold and a world record in the event, defeating an American team anchored by Katie Ledecky on the first day of swimming competition at Olympic Aquatics Stadium.

The Australians finished in 3 minutes, 30.65 seconds, breaking the record of 3:30.98 they set two years ago. Simone Manuel and Abbey Weitzeil gave the U.S. an early lead, but Campbell's sister, Bronte, overtook Dana Vollmer as they approached the 300-meter mark. Cate Campbell — the world record holder in the 100 free — held off Ledecky, the distance superstar who swam the relay's anchor leg in both the preliminaries and the final.

The U.S. finished second in 3:31.89, and Canada was third in 3:32.89.

Two other world records were broken Saturday. Katinka Hosszu of Hungary demolished the mark in the women's 400 individual medley, winning in 4:26.36. Adam Peaty of Great Britain lowered the record in the men's 100 breaststroke, clocking a time of 57.55 in the preliminaries.

"Definitely my favorite race ever,'' Bronte Campbell said of the relay. "In an Olympic final, with my sister and two girls I've known since I was 12 or 13, and a gold and a world record. You can't ask for more.''

Mack Horton also won gold for Australia in the men's 400 freestyle, giving Australia two gold medals after one day in Rio — double the total it won in a forgettable performance at the 2012 London Games. The U.S. finished with three silver medals Saturday, from the relay team, Maya DiRado (women's 400 IM) and Chase Kalisz (men's 400 IM).

Japan's Kosuke Hagino broke the U.S. hammerlock on the men's 400 individual medley, ending a U.S. streak of five Olympic titles in the event. Michael Phelps, who won in 2004 and 2008, did not compete in the 400 IM this year. Ryan Lochte, the winner in 2012, did not qualify for the Olympics in that event.

One night after getting the Olympic party started at the Opening Ceremony, Rio kept the good vibes going at the Olympic Aquatics Stadium. This is a city that comes alive after dark, and it embraced the late-night ­schedule for the swim finals.

A line of samba dancers — complete with long feather headdresses — turned the pool deck into a nightclub during the swimmers' warmups, moving to the pulse of a drum corps that got the entire arena swaying. The exuberant public-address announcer, speaking in Portuguese, sounded as if he would break into a bellow of "Goooaaaal!'' any minute.

The Brazilian fans were swept up in emotion any time one of their own competed. During the semifinals of the men's 100 breaststroke, when they had one countryman in each heat, they had to be asked to quiet down for the start of the race. When they didn't — a major breach of swim etiquette — the rest of the crowd shushed them loudly.

The Americans won their 22nd Olympic medal in the women's 4x100 free relay, but they have not taken gold since 2000. The Aussies prevailed in 2004 and 2012, and the Netherlands won in 2008.