ROME: Michael Phelps defied the odds to deliver another crushing defeat to Milorad Cavic on Saturday, besting the Serbian yet again in the 100m butterfly World Championships final.

Phelps smashed the 50-second barrier with a time of 49.82sec, and handed Cavic a bigger defeat than the one he suffered by a fingertip at the Beijing Olympics.

Cavic lost by one-hundredth of a second to Phelps in the 100m fly at the Beijing Games and had hoped to turn the tables on the American superstar in Rome.

Phelps was having none of it.

"It was an incredible race, we all went a lot faster than I expected," said Cavic, who clocked 49.95sec.

As the crowd strained to watch them, Cavic made the most of the moment, taking up a stance facing Phelps across the blocks prior to the start.

Phelps, famous for rising to the big occasion, felt the tension as well.

Cavic appeared perfectly placed to deal Phelps his second defeat of these championships.

He grabbed Phelps's world record with a time of 50.01sec in the semi-finals, and races in one of the new generation polyurethane swimsuits that have seen 39 world records fall with one day remaining.

Despite a long post-Olympic layoff, and patchy preparation, Phelps has accounted for two of those records, also notching a world mark in the 200m fly final.

He also has two relay golds, and had one more relay yet to come, but the showdown with Cavic was in a class by itself.

As in Beijing, Cavic used his early speed to take the lead at the turn.

Phelps was fourth at the 50m mark - better than his seventh in Beijing - and used his vaunted finishing strength to overhaul Cavic in the final 25m.

Cavic said his race strategy was perfect, but there was no strategy that can account for Phelps. LINK..