Michael Phelps is used to celebrating 200 meters butterfly victories but the American's gold medal at the world swimming championships on Wednesday was particularly sweet.

The win not only came one year ahead of the London Olympic Games but also helped atone for a series of recent losses in what has been his signature event for the best part of a decade.

After the race, the relief was palpable on both his face and that of his mother Debbie, who was cheering Phelps on from the stands.

So dominant was the Olympic champion, he had not been beaten in a 200 butterfly race for nine years until he lost to China's Wu Peng twice and Australia's Nick D'Arcy earlier this year.

D'Arcy did not make the Australian team after falling ill at the time of the national championships in March but Wu loomed large in lane two, right next to Phelps on Wednesday.

The American, however, exploded off the blocks and led for the first 100 meters before Takeshi Matsuda had a phenomenal turn and the pair raced stroke-for-stroke down the third lap in which the Japanese swimmer took a narrow lead.

Phelps then did exactly what Matsuda had done 50 meters earlier and his turn for home propelled the American back in front and he held on to win in one minute, 53.34 seconds.

Matsuda took the silver in 1:54.01 while Wu, roared on by the home crowd, grabbed bronze in 1:54.67.

"The race felt good," Phelps told reporters. "I wanted to do what I usually do when I am in a better shape. I wanted to step on it from the get-go.

"I saw other swimmers when I did the 150, I wanted to finish it hard and I put my hips into it."

Phelps has already won a bronze in the 4x100 freestyle relay and silver behind compatriot Ryan Lochte in the 200 freestyle, and is to swim in the semi-finals of the 200 individual medley later on Wednesday.