Michael Phelps will need to rely on the same turn of speed that brought him home in the semi-finals on Friday if he is to retain his 100 meters butterfly title at the world swimming championships on Saturday.

On Friday, the Beijing Olympic champion had hit the turn at 50 meters, languishing in fifth place in his semi, before he rocketed to the fastest qualifying time of 51.47 seconds.

The 26-year-old Phelps is using the championships in Shanghai to begin his preparations for his final Olympics in London next year.

But he will not face long-time rival Milorad Cavic in Saturday's final after the Serb struggled home in 52.67 in the heats and did not qualify for the semi-finals.

Brazil's Cesar Cielo will also be facing a tight tussle to retain his title in the final of the men's 50 freestyle after compatriot Bruno Fratus pipped the Olympic champion by 0.03 seconds to the fastest qualifying time.

"I will just have to get a better race, a better start, to get the last 10 meters a little better," Cielo said. "It's gonna be better to have my Brazilian team mate by my side.

"He is the favorite for the gold. It's good to have someone fast by your side. I feel confident. I will just have to work as much as I can with what I have right now."

Britain's Rebecca Adlington, the Beijing Olympic champion, will start a slight favorite to win the women's 800 freestyle title after she held off the challenge of Denmark's Lotte Friis in qualifying.

Friis, the 1500 freestyle champion in Shanghai, pressured the 22-year-old Briton for the final 200 meters of their heat before Adlington kicked away on the final length to win by almost a second.

American teenager Missy Franklin could also win her first individual world title in the women's 200 backstroke after Beijing Olympic champion Kirsty Coventry failed to make the final.

The 16-year-old Franklin, who stands at 1.85 meters tall (6'1"), was fastest qualifier by almost two seconds.