The 24-year-old American erased any concerns he lacked motivation after he won eight gold medals at the Beijing Olympics, ending compatriot Ian Crocker's six-year grip on the 100 butterfly record.

Phelps' power was on display when he went out fast and pulled away after the turn clocking 50.22 seconds, leaving Tyler McGill a distance second in 51.06.

Phelps now holds five individual world records -- 200 and 400 individual medleys, 100 and 200 butterfly and 200 freestyle.

He had previously held the 100 butterfly record when he broke the mark in the semi-finals of the 2003 Barcelona world championships.

Crocker, however, broke that record with victory in the final the next day and has lowered it twice since -- the last time at the world championships in 2005 when he swam 50.40.

"It was big, it was something I really, really wanted," Phelps told reporters. "I've wanted that record ever since he took it at the '03 worlds.

"He just smashed me in the 100 at the '05 worlds, I remember saying then, 'I never want to get beaten that bad again'.

"That was a wake up call for me."

Phelps' display also confirmed he is back to full fitness after a tumultuous six-month layoff following Beijing, which included a three month suspension from USA Swimming when he was photographed inhaling from a pipe used for smoking marijuana.

He also gained seven kilograms in weight and contemplated retirement but is now looking to add to his 17 world championships gold medals later this month in Rome.

He has already qualified for three events for Rome with victories in the 200 freestyle, 100 and 200 butterfly and is racing the 100 freestyle on Friday.

Phelps has not been satisfied with his overall performance, particularly in the 200 metre events where he posted year best times in winning both, but failed to produce personal bests.

A beaming Phelps, however, and coach Bob Bowman could find no fault with Thursday's performance.

"I think he sensed he was in condition to break the record," said Bowman. "He was really fired up to do it.

"I think it means a lot to him because in the race when Ian broke that record (in 2005), that was the worst Michael has ever been beaten.

"That was an amazing record, that's why it stood the test of time."

Evergreen Dara Torres, winner of three silver medals in Beijing, continued to defy her age touching first in the women's 50 freestyle in a time of 24.43 seconds.