ROME: China won the women's 4x100m medley relay world title in a world record time of 3min 52.19sec here on Saturday.
China broke the previous record of 3:52.69 set by Australia in winning Olympic gold in Beijing last August.
"It's so cool. We had a great race. We all improved our times by one second," said Zhao Jing, whose opening backstroke leg left China in second by a narrow margin.
Chen Huijia gave her team a narrow lead on the second breaststroke leg, and they held off a battling Aussie squad to keep the advantage the rest of the way.
"We all swam so well, we all did a great job today," said butterflyer Jiao Liuyang. "We weren't paying attention to the other teams we were too far from them. We concentrated on our own race."
Li Zhesi finished off the closing freestyle against former world record-holder Libby Trickett as Australia took silver in 3:52.58 - also under their Beijing world record.
"It was a tough competition, even without the US team, but we put together the race and we did our best," Trickett said.
The United States sensationally failed to qualify for the final, posting the 10th-fastest time in the heats.
It's the first time in the history of the World Championships that the Americans failed to claim either first or second in the event. Bronze went to Germany in 3:55.79.
It was China's fourth swimming gold of the championships, a haul that included the first world swimming title for a man, Zhang Lin in the 800m free.