Greatest swimmer Michael Phelps aims high in his last Olympics

Michael Phelps who is arguably the greatest Olympian of all time leads the mighty United States into battle in the Olympics against traditional rivals Australia.

Phelps, whose record 22 Olympic medals include 18 gold, caps his career with a fifth Games appearance in Rio. But just as he's far from invulnerable at 31, so the United States could find itself hard-pressed to match their 2012 haul of 16 gold medals, nine silver and six bronze.Australia, in particular, are aiming to rebound from a shocking London Games -- their first since 1976 without an individual gold.

Australian coach Jacco Verhaeren, however, said the 2016 squad's mission has nothing to do with atonement. "I don't think we're here to make up for any disappointment," he said. "Nobody is busy with what happened or what might happen." Australia's Cate Campbell arrives in Rio on a high after breaking the long-standing women's 100m freestyle world record in July, while Cameron McEvoy is a gold medal contender in the men's 50m and 100m free.

"It's always special to stand up against the Americans," said McEvoy. "That mutual respect gets the adrenaline pumping." But swimmers from around the globe are set to challenge the twin powerhouses when eight days of competition kicks off on Saturday. 

China will be keen to at least maintain the second place in the swimming medals table they achieved for the first time in London with five golds. Sun Yang, looking to defend his 2012 gold in the 400m and 1,500m freestyles, heads a Chinese squad that also includes Ning Zetao, 2015 world champion in the men's 100m free and Ye Shiwen, reigning champion and world record-holder in the women's 400m individual medley.

Hagino Kosuke leads the Japanese charge. He's aiming to wrest the 400m individual medley gold from US hands, but he'll have competition in the same event from compatriot Daiya Seto. South Africa's Chad le Clos -- who bested Phelps in the 200m butterfly in London -- is back to try to hold off a Phelps bent on revenge.

"The sport has changed," Phelps said of the need to be wary of challengers from all directions. Britain boasts medal contenders in freestyler James Guy and breaststroker Adam Peaty and Hungarian Laszlo Cseh tops the world rankings in the 100m butterfly -- one of two events Phelps is trying to win for a fourth straight time. Hungary's "Iron Lady" Katinka Hosszu is eyeing a medley double -- with perhaps a backstroke or butterfly strike thrown in.