Olympics: Aussie teen stars as Ledecky motors on

Australia's new teenage swimming sensation Kyle Chalmers won the blue riband Olympic 100m freestyle gold, as golf headed for a controversial return to the Games after 112 years, AFP reports.

Chalmers, only 18, became the first Australian in 48 years to win the prestigious race.

The teenager hit the wall in 47.58 seconds, just 22 hundredths ahead of Belgium's Pieter Timmers and American Nathan Adrian, the 2012 champion, who took bronze.

Chalmers is the first Australian to win the 100m free since Mike Wenden in Mexico City in 1968. But he did not realize it had been so long.

"I actually had no idea about that to be honest with you," he said. "I'm someone that probably doesn't follow swimming too much.

"I kind of stay away and follow basketball and soccer and football. I know that we've been beaten a few times."

His victory came as American Katie Ledecky sealed her third gold medal of the Games, anchoring the United States team home in the 4x200m relay.

She already has the 200m and 400m freestyle titles in Rio.

Elsewhere in the pool, Michael Phelps returned from his heroics in winning his record 20th and 21st gold medals, to safely reach the final of the 200m individual medley on Thursday.

- Kazakhstan double -

There was also a rare gold medal for Kazakhstan in the 200m breaststroke, where Dmitriy Balandin won his country's first Olympic swimming title since the break-up of the Soviet Union.

It was one of two golds for Kazakhstan on the fifth full day of Olympic action. Kazakh weightlifter Nijat Rahimov won the other in the men's 77kg category.

While swimming remains one of the powerhouse Olympic sports, golf returns on Thursday for the first time since the 1904 Games in St Louis, Missouri when George Lyon of Canada won the men's title.

It is coming back now already facing doubts about the future because top players -- including the world's leading four, Jason Day, Dustin Johnson, Jordan Spieth and Rory McIlroy -- have stayed away.

They have cited the Zika virus and scheduling concerns. But some on the International Olympic Committee suspect indifference and IOC president Thomas Bach has said the absence of star names will be one criteria looked at when it comes to deciding which sports keep their Olympic places.

Gymnastics is assured of its place for Tokyo 2020 and Japanese star Kohei Uchimura underscored his gymnastics dominance as he clinched a second straight all-around gold medal on Wednesday.

Uchimura snatched victory by just 0.099 points with a daredevil routine on the horizontal bar.

"I remained calm and controled. This calmness I think was the key to my success," Kohei said.

The men's rugby sevens saw a day of upsets, with New Zealand losing to Japan in the groups before being eliminated in the quarter-finals by Fiji, who are chasing their country's first Olympic medal. Japan scored a shock win over France to reach the last four.

In fencing, Tunisia's Ines Boubakri became the first African woman to win a medal in the sport with a bronze in the individual foil, dedicating her achievement to "the Arab woman... who has her place in society."

Her French husband Erwann Le Pechoux could get a medal in the individual foil event.

In football, Gabriel Barbosa struck twice as hosts Brazil finally sparked into life with a 4-0 win over Denmark to secure a place in the quarter-finals against Colombia.

Veteran cyclists Fabian Cancellara and Kristin Armstrong hammered home their Olympic superiority with wins in the men's and women's time trial.

It was Armstrong's third consecutive win in the race, a day before her 43rd birthday.

"When you've already been two times at the pinnacle of the sport, why risk coming back for the gold medal? The best answer I can give is that I can," said Armstrong, who retired after each of her two previous Olympic wins.

The star-studded USA men's basketball had to dig deep against Australia before pulling away for a 98-88 win.

Carmelo Anthony scored 31 points and Kyrie Irving added 19, including 12 in the final period as the Americans subdued their rivals.