Members of the men's 4 x 200m freestyle relay team, Michael Phelps, Conor Dwyer, Ryan Lochte and Ricky Berens (L-R) hold their national flag as they celebrate their gold medal win in the men's 4x200m freestyle relay victory ceremony during the London 2012 Olympic Games at the Aquatics Centre July 31, 2012. Phelps won the record 19th Olympic medal on Tuesday when he joined forces with his U.S. team mates to win the 4x200m freestyle relay. REUTERS/Jorge Silva
Michael Phelps of the U.S. poses with his gold medal after winning the men's 4x100m medley relay final during the London 2012 Olympic Games at the Aquatics Centre August 4, 2012. Phelps ended his incredible Olympic career on the perfect note on Saturday, winning his 18th gold medal for the United States in the men's medley relay, the last time he will swim a competitive race. Swimming the butterfly leg, the most decorated Olympian of all time went out as the ultimate winner when he joined forces with Matt Grevers, Brendan Hansen and Nathan Adrian to crush their opponents and win the gold. REUTERS/Jorge Silva
Michael Phelps (C) of the U.S. swims the butterfly stroke during his team's victory in the men's 4x100 meters medley relay at the National Aquatics Center during the Beijing 2008 Olympics August 17, 2008. Picture taken with underwater camera. REUTERS/Wolfgang Rattay
Michael Phelps of the U.S. competes in the men's 400m individual medley swimming final at the National Aquatics Center during the Beijing 2008 Olympic Games August 10, 2008. Picture taken with underwater camera. REUTERS/Wolfgang Rattay
Michael Phelps and Garrett Weber-Gale (L) of the U.S. celebrate after winning the men's 4x100m freestyle relay swimming final at the National Aquatics Center during the Beijing 2008 Olympic Games August 11, 2008. REUTERS/David Gray
Michael Phelps of the U.S. looks down at the pool after his team was beaten to the gold by France in the men's 4x100m freestyle relay final during the London 2012 Olympic Games at the Aquatics Centre July 29, 2012. REUTERS/David Gray
Photographers surround Michael Phelps of the U.S. after he won his eighth gold medal of the games at the National Aquatics Center during the Beijing 2008 Olympic Games, August 17, 2008. REUTERS/Gary Hershorn
A combination photo shows Michael Phelps of the U.S. holding each of his eight gold medals in the swimming competition at the Beijing 2008 Olympic Games. Phelps won his eighth gold medal on Sunday, beating Mark Spitz's record of seven gold medals won at a single Games in 1972. The medals were won in: (Top row L-R) 400m individual medley (August 10), 4x100m freestyle relay (August 11), 200m freestyle (August 12), 200m butterfly (August 13). (Bottom row L-R) 4x200m freestyle relay (August 13), 200m individual medley (August 15), 100m butterfly (August 16), and 4x100m medley relay (August 17). REUTERS/Staff
Gold medallist Michael Phelps of the U.S. listens to his national anthem during the men's 200m individual medley victory ceremony during the London 2012 Olympic Games at the Aquatics Centre August 2, 2012. REUTERS/David Gray
Michael Phelps of the U.S. holds up his award recognising him as the most decorated Olympian, during the London 2012 Olympic Games at the Aquatics Centre August 4, 2012. Phelps ended his incredible Olympic career on the perfect note on Saturday, winning his 18th gold medal for the United States in the men's medley relay, the last time he will swim a competitive race. REUTERS/Brian Snyder
Michael Phelps' Olympic history
Michael Phelps, the most decorated Olympian of all time with 18 gold medals among his career haul of 22, will carry the U.S. flag at the Rio Olympics opening ceremony.
