Australia's Campbell fires 50m freestyle warning

ROME (Reuters) - Australia's Cate Campbell fired a warning to her rivals in the women's 50 meters freestyle by setting the pace in Saturday's heats at the world championships.

Campbell clocked a championship record of 24.24 seconds to win her heat and qualify for the semi-finals with a faster time than her compatriot and 2007 champion Libby Trickett, Dutch world record holder Magdalena Veldhuis and Germany's Olympic champion Britta Steffen.

In a big surprise, the United States failed to reach the women's 4x100 medley relay final.

With some top swimmers rested, the silver medalists at the last four world championships managed only second place in their heat behind Denmark and 10th overall.

China posted the fastest time by winning their race in three minutes 56.14.

World and Olympic champions Australia only just qualified in seventh after the team led by Stephanie Rice came third in a heat won by Britain.

China's Sun Yang beat Tunisia's Olympic champion Oussama Mellouli by two hundredths of a second to qualify for Sunday's 1,500m freestyle final with a fastest time of 14:54.54.

Zhang Lin, who became China's first male long-course swimming world champion on Wednesday when he destroyed the world record to win the 800 freestyle, also went through when he finished third in a heat won by Canada's Ryan Cochrane.

"Doing 1,500 always hurts," Mellouli, who took silver in the 400 and 800 earlier this week, told reporters.

"The Chinese guy did a great job in the 800. I've had two silver medals so far, let's hope I win tomorrow."

Russia's Yuliya Efimova came within 0.01 of a world best when she went through to the women's 50 meters breaststroke semi-finals with a championship record of 30.24 seconds.

American Rebecca Soni, who won the 100m breaststroke title on Tuesday, came home first in her heat to qualify seventh overall with Canadian world record-holder Amanda Reason also through.

After winning the men's 200 backstroke final with a world record on Friday, America's Aaron Peirsol crashed out of the heats of his less-favored 50.

Brazil's Guilherme Guido topped the time sheets with a championship record of 24.49 while other big names such as Japan's new 100 backstroke champion Junya Koga, his compatriot Ryosuke Irie, Germany's Helge Meeuw and Spain's Aschwin Wildeboer all went safely through.