 "Michael Phelps bids for astonishing 20th gold")
Michael Phelps will set his sights on a record-extending 20th Olympic swimming gold today when he tries to wrest back the 200m butterfly title from Chad le Clos.
Katie Ledecky will also be battling for another key swimming title on an Olympic day in which 14 golds will be up for grabs.
Phelps, the most decorated Olympian of all time has already topped the podium in Rio after the USA's 4x100m win, and will now go for individual glory at the age of 31.
Phelps won his signature event in 2004 and 2008 before his finger-tip loss to le Clos in 2012, and he will be desperate to regain the gold in his final Games.
Ledecky and Sarah Sjostrom, both already world record-breakers at this Games, will duel for the 200m freestyle title in another hotly anticipated race.
"It was a tough race, I knew she was going to go fast. It was about getting my lane for tomorrow," said Ledecky, after finishing behind Sjostrom in Monday's semi-finals.
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Ledecky set a world record as she won the 400m freestyle and the 19-year-old American is gunning for the first 200m, 400m and 800m treble in 48 years.
Read our full coverage on the 2016 Rio Olympics
Read our full coverage on the 2016 Rio Olympics
In the 200m individual medley, Hungary's "Iron Lady" Katinka Hosszu is going for her third gold medal after she smashed the world record in the 400m race and followed up by winning the 100m backstroke on Monday.
Elsewhere Simone Biles will go for the first Olympic gold of her already trophy-laden gymnastics career as the United States bid to defend their team title.
The tiny Texan powered a dominant United States in Sunday's preliminaries with the reigning champions crushing their nearest rivals China by nearly 10 points.
The three-time all-around champion was too young to compete in London but since has won a record 10 world titles and now just needs an Olympic gold to put her name among the legends.
A team gold would also put Biles on the road to the record haul of five gold medals the 19-year-old covets in Rio. She also topped the all-around, floor, beam and vault in preliminaries.
"We just have to stay confident and do our routines like we normally do," said a matter-of-fact Biles.
In diving, China's Chen Ruolin will attempt to match team-mate Wu Minxia's record of five gold medals when she starts as favourite with partner Liu Huixia in the synchronised 10m platform.
Gold medals are also on offer in canoe slalom, equestrian, fencing, weightlifting, judo and shooting.