Michael Phelps broke his own 200metres butterfly world record at the Missouri Grand Prix in Columbia.
The 21-year-old swam in a time of one minute 53.71 seconds to take nine-hundredths of a second off the previous world best he set at the Pan Pacific Swimming Championships last August in Canada.
He said: 'When I saw the time my reaction changed and I basically went nuts. I did not expect that.
'I didn't think I was going to break a record but I could tell something good was happening when I heard the crowd.'
Phelps, who claimed six gold medals at the 2004 Olympics, will compete at next month's World Championships in Melbourne.
