RIO DE JANEIRO (AFP) - Swimming and the debut of rugby sevens headline the opening day of the Rio Olympics on Saturday (Aug 6) when 12 gold medals will be decided.
In the pool, Chinese star Sun Yang defends his 400m freestyle title but faces a stern test from Australia's Mack Horton who has the top time of 2016.
The event also features South Korea's 2008 Olympic champion Park Tae-Hwan, who is back from an 18-month steroid ban to compete in his third Games.
The first gold at the pool will come in the men's 400m individual medley.
Japanese team-mates and good friends Daiyo Seto and Kosuke Hagino meet in a race which will not feature American stars Michael Phelps and 2012 gold medallist Ryan Lochte.
Hungary's Katinka Hosszu is the favourite in the women's 400m individual medley, a race won by China's Ye Shiwen in world record time four yeats ago in London.
Defending champions Australia target a world record with sisters Cate and Bronte Campbell making up half the squad 4x100m relay squad.
Despite the duels in the pool dominating the opening day, the first gold of the 2016 Games will come at the shooting range in the 10m women's air rifle event.
China's Yi Siling is defending her title, In the men's 10m air pistol, South Korea's Jin Jongoh can become the second shooter after Wang Yifu of China to win the event in two Olympic Games.
Two-time Tour de France champion Chris Froome leads the Great Britain team as he bids to emulate the feat of decorated compatriot Bradley Wiggins in the cycling road race.
Four years ago, Wiggins followed up becoming the first ever Briton to win the Tour by adding Olympic gold in London.
Golds will also be decided in women's weightlifting in the 48kg class, men's team archery, women's epee fencing and in judo where the men's 60kg and women's 48kg titles will be won.
Rugby sevens makes its Games debut with women's teams from trans-Tasman rivals Australia and New Zealand fancied to battle it out with Canada and Britain for gold.
The three-day tournament, to be followed by the men's competition, showcases 12 sides.
It will be the first time since 1924 that rugby has been played at the Olympics and the debut for sevens, the previous four incarnations featuring 15-a-side tournaments.
Top seeds Australia will play the United States, Fiji and Colombia in Pool A while New Zealand, France, Spain and Kenya contest Pool B. Canada, Britain, Brazil and Japan complete the field in Pool C.
Also getting under way on Saturday is men's basketball, where defending champions United States take on China.
US teams are 130-5 in the Olympics, failing to win the gold medal on only three occasions in 17 attempts.
The five defeats include a controversial last-gasp loss to the Soviet Union in 1972, 1988 to the Soviets to spark creation of the original NBA "Dream Team", and three times in 2004.