US swimmer Michael Phelps poses after receiving the trophy of the greatest olympic athlete of all time after winning gold in the men's 4x100m medley relay final during the swimming event at the London 2012 Olympic Games on August 4, 2012 in London.

London 2012 Olympic Games on August 4, 2012 in London. Phelps brought his Olympic swimming career to a close in style, winning a record 18th gold medal as the United States stormed to victory in the 4x100m medley relay.

US swimmer Michael Phelps (C) reacts with teammates Brendan Hansen, Nathan Adrian and Matthew Grevers after they competed in the men's 4x100m medley relay final during the swimming event at the London 2012 Olympic Games on August 4, 2012 in London.

Michael Phelps of the U.S. swims the butterfly leg of the men's 4x100m medley relay final during the London 2012 Olympic Games at the Aquatics Centre August 4, 2012.

Michael Phelps of the U.S. celebrates winning gold in the men's 100m butterfly final during the London 2012 Olympic Games at the Aquatics Centre, August 3, 2012.

United States' Michael Phelps displays his gold medal for the men's 100-meter butterfly swimming final at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, Friday, Aug. 3, 2012. (AP Photo/Matt Slocum)

United States' Michael Phelps poses with his gold medal for the men's 4x200-meter freestyle relay swimming final at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, July 31, 2012.

From left, Michael Phelps, Conor Dwyer, Ricky Berens and Ryan Lochte pose with their gold medals for the men's 4x200-meter freestyle relay swimming final at the Aquatics Centre in London, July 31, 2012.

Michael Phelps competes in the men's 4x200-meter freestyle relay swimming final at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, July 31, 2012.

Michael Phelps waits to receive his silver medal for the men's 200-meter butterfly swimming final at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, July 31, 2012.

Michael Phelps displays his eighth gold medal after the men's 4x100-meter medley relay final at the Beijing 2008 Olympics in Beijing.

Michael Phelps competes in the 100-meter butterfly at the Indianapolis Grand Prix swimming meet in Indianapolis, March 29, 2012.

Michael Phelps and his mother Debbie Phelps attend the Heart Truth's Red Dress Collection during Fashion Week in New York, February 8, 2012.

Olympic gold medalist Michael Phelps posing in the Belmont Plaza Olympic Pool in Long Beach, Calif. (Jan 2008 file photo)