Michael Phelps became the first man to dip under 50 seconds for the 100metres butterfly when he retained his title at the World Championships.

Victory prompted an outpouring of emotion and aggression by the American, but Phelps had appeared to be vulnerable coming into the race especially given Milorad Cavic had lowered his world record yesterday.

The 14-time Olympic champion was more than half a body-length down at the turn, in fourth, but he moved through the field to touch out the Serbian in 49.82seconds.

By so doing Phelps responded to Cavic's assertions that the American should take immediate delivery of an Arena X-Glide suit instead of remaining loyal to Speedo.

His victory in the most absorbing race of the championships so far prompted the 24-year-old into sitting on the lane rope, pumping his arms, spitting water, beating his chest and pointing exaggeratedly at his suit.

Phelps said: "A bunch of friends at home texted me last night and this morning telling me this race was mine and that I had to go for it.

"I knew if I went out from the turn 23.4 or 23.5 seconds I would be in with a chance.

"I don't think I have ever been that emotional or pumped up after a race. You saw by my reaction how much I wanted that.

"I know comments have been made in the past and I welcome them. Comments motivate me and I let my swimming do the talking."

His coach Bob Bowman welcomed the return to textile suits after previously describing the situation as a "shambles".

Of the race, Bowman said: "It was perfectly done. He showed speed he had never done before."

Bowman also revealed Phelps would return to competing in the 200m individual medley.

Phelps' effort was one of four world records at the Foro Italico, bringing the total to 39 with one day of competition remaining.

Kirsty Coventry demolished her own mark in the 200m backstroke and, like Phelps, was clad in a Speedo.

The Zimbabwean had come eighth over 100m and fourth in the 200m individual medley but she led from start to finish to win in 2:04.81, 0.43 seconds inside her old record.
Russia's Anastasia Zueva was second and Elizabeth Beisel of America took bronze.

Lotte Friis was a surprise winner of the 800m freestyle ahead of Britain's Jo Jackson, who claimed her third medal of the championships, and Alessia Filippi. Olympic champion Rebecca Adlington was fourth.

Friis, of Denmark, won in 8:15.92, the second fastest time in history behind Adlington's world record set in Beijing last year.

Cesar Cielho Filho added the 50m freestyle title to the 100m ahead of world record holder Frederick Bousquet and fellow Frenchman Amaury Leveaux.

China's women won the 4x100m medley relay crown in a world record of 3:52.19, with Australia and Germany making up the minor placings, and Marieke Guehrer triumphed in the 50m butterfly.

Liam Tancock, clad in an Adidas hydrofoil, set a new world record of 24.08 in the 50m backstroke semis while Australian pair Sarah Katsoulis and Cate Campbell were fastest through in the 50m breaststroke and 50m freestyle respectively.