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 (BRITAIN - Tags: SPORT SWIMMING OLYMPICS HEADSHOT TPX IMAGES OF THE DAY)