Rio de Janeiro, Brazil: American legend Michael Phelps joins the fray in Rio Sunday hoping to reverse the trend after triumphant Australia dominated the first day’s action in the Olympic pool.

Inspired by swimmers Mack Horton and Cate Campbell, Australia surged to the top of the medals table after an intense first day of competition.

That laid down the gauntlet for the Americans and now all eyes are on Phelps, 31, who made his Olympic debut in 2000 and admits it is getting no easier to push his aging frame to the limit once more in search of a potential six more titles.

His campaign is to begin in Sunday night’s 4x100m freestyle relay final where the chief rivals will again be the Australians who showed superb form Saturday.

Aussie Horton dethroned China’s Sun Yang to win the men’s 400m freestyle and then Campbell, the individual world record-holder, anchored Australia to 4x100m freestyle gold in 3min 30.65sec, the fastest time in history.

“Definitely my favorite race ever, in an Olympic final with my sister and two girls I have known since I was 12, 13 years old and a gold and a world record,” said Campbell, who swam with Emma McKeon, Brittany Elmslie and her sister Bronte.

That was one of a trio of swimming world records that marked the day as well as a first-ever Olympic gold for Vietnam that triggered rejoicing back home.

Britain’s Adam Peaty was the early star in the swimming pool. He set an eye-popping new world record of 57.55sec in the men’s 100m breaststroke heats—his first-ever Olympic swim. He challenges for gold in the final later Sunday.

Hungary’s Katinka Hosszu shattered the world record by more than two seconds to win the women’s 4x100m individual medley.

There was joy also for teenage American shooter Ginny Thrasher, who upset China’s champions in the women’s 10m air pistol to snare the first gold of the Games.

Belgium’s Greg van Avermaet won a crash-filled men’s cycling road race, and rugby returned to the Games after a gap of 92 years.

The results pushed Australia to the top of the early medals table and confirmed their return to prominence in swimming, after their poor outing at London 2012.

Phelps and compatriot Katie Ledecky will hope to test that dominance with Ledecky, the 800m freestyle title holder, competing for three individual golds.

Stray bullet -The intense action in the competition arena on the opening day was accompanied by unwanted action of a different sort as a military bomb squad detonated a suspicious package near the finish of the men’s cycle race at Copacabana beach, causing jitters among onlookers.

At the equestrian venue at Deodoro, which is on a military base, a stray bullet ripped through the media center’s canvas roof.

“All authorities assured that the bullet was a stray bullet that has nothing to do with the Games or the Olympic family or the press. The area was not a target,” said Games spokesman Mario Andrada.

Later Hoang Xuan Vinh wrote his name in the history books when he won the men’s 10m air pistol, claiming the first ever gold medal for communist Vietnam. All the sweeter for the Vietnamese as he beat a Chinese rival Pang Wei into third place.

“So proud! But the greatest happiness was that we won over China,” Nguyen Cao Ky Duyen, a Vietnamese music show host based in the US but popular in her homeland, wrote on Facebook.

Thailand’s Sopita Tanasan won the first weightlifting gold, the women’s -48kg, with a total lift of 200kg.

Scandal-hit Russia, under fire over a massive doping scandal, got off the mark when Beslan Mudranov won the men’s 60kg judo.

French gymnast Samir Ait Said suffered Olympic agony. His lower left leg snapped and skewed to a gruesome angle when he landed awkwardly from a vault.

Yusra Mardini, the Syrian refugee who was swimming for her life to a Greek island less than a year ago, won her 100m butterfly heat but failed to reach the semi-finals.

“Everything was amazing. It was the only thing I ever wanted was to compete in the Olympics,” said the 18-year-old, who is representing the Games’ first ever refugee team.