ROME: Michael Phelps won the men's 100m butterfly world title here on Saturday, again subduing Beijing Olympic rival Milorad Cavic to win in a world record 49.82sec.

The US superstar had beaten Cavic by just one one-hundredth of a second in Beijing, and the Serbian turned this World Championships showdown into a grudge match.

Cavic looked to have the upper hand heading into the final. He broke Phelps's world record in the semis with a time of 50.01sec, and races in one of the new generation polyurethane swimsuits that have seen 37 world records fall so far this week.

But Phelps notched his second world mark of the meet - and captured his fourth gold - without benefit of the newest technology, becoming the first man to break the 50-second barrier in the once-revolutionary but now outdated Speedo LZR.

Cavic also broke 50 seconds, taking silver in 49.94. Spain's Rafael Munoz was third in 50.41.

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.

Phelps added the title to his 200m butterfly crown and 4x100m and 4x200m freestyle relay golds won in Rome.