Ian Crocker lowered his own world record for the 100 metres butterfly at the U.S. Olympic Swimming Trials on Tuesday and said he was aiming to race under 50 seconds at the Olympic Games.

Crocker's time of 50.76 seconds broke the previous world mark of 50.98, which he set in winning gold at the 2003 World Championships in Barcelona.

"Going under 50 seconds is definitely my goal. I believe it's possible to swim a 49-second 100-metre fly and I expect it will be done within a short period of time," Crocker said.

Racing against teenage wonder boy Michael Phelps, Crocker took the lead at the start and never relinquished it, splitting 23.62 seconds at the 50-metre turn.

"When you are racing against Michael nothing is given," Crocker said. "He's a phenomenal swimmer and he is trying to do something special.

"I'm trying to be the best I can be and tonight was an example of that. Our rivalry brings out the best in me."

Crocker's celebration was subdued after setting the world record and he said: "I wanted to go a world record here to set me up for Athens, so my feeling was one of relief.

"I haven't felt that good all week here as I am just starting my taper. I was reserved because I know I'm only at 90 percent and there's much more to go.

Phelps, who is looking to surpass Mark Spitz's 1972 record of seven Olympic gold medals at next month's Athens Games, said: "It was not my best race.

"I knew he got too far ahead of me. I knew it was going to take a world record. Hopefully, in five weeks we can change a few things."