(CNN) -- Michael Phelps claimed his third gold of the world swimming championships in Shanghai Saturday but then admitted he was short of peak fitness.
The American superstar won the 100m butterfly ahead of Poland's Konrad Czerniak and compatriot Tyler McGill in 50.71 seconds.
It was his sixth medal of the meet in his return to China where he took an unprecedented eight gold medal tally in the 2008 Beijing Olympics.
Phelps, who has been overshadowed by teammate Ryan Lochte in Shanghai, said he would be looking to improve with next year's Olympics in London his target.
"I'm not in the right shape physically. I want to be faster. It's a tough race," Phelps told AFP.
In other action Saturday, controversial Brazilian star Cesar Cielo earned his second sprint title in the men's 50m freestyle with a winning time of 21.52 seconds.
Olympic champion Cesar was only cleared to compete in the championships when a special hearing of the Court of Arbitration in Sport let him off with a warning after he tested positive for a banned diuretic in May.
His appearance at the championships has drawn criticism for fellow swimmers and Cielo admitted it had taken its toll.
"For now, it's a big relief for me. I've had a crazy month and it's good to finish with a gold," he told AFP.
"I think that shows a lot of mental control. I think I became a more mature person. Even though I'm 24, I feel like I'm 44 right now. It was definitely the biggest challenge of my life."
Missy Franklin of the United States, who is only 16, showed she will be a force at the London Games with another two gold medals.
Franklin took the women's 200m backstroke from Australian Belinda Hocking and then helped the U.S win their first 4x100m medley in more than a decade.
Britain's double gold medal winner from the Beijing Games, Rebecca Adlington took her first world title as she edged out Lotte Friis of Denmark in the final length of a thrilling women's 800m freestyle with America's Kate Ziegler third.
Dutch swimmer Inge Dekker was an upset winner of the women's 50m butterfly to claim her first individual world title.