American Natalie Coughlin, the two-times Olympic champion for the 100 backstroke, had turned for home under world record pace but was swum down by the 20-year-old Zhang and Russia’s Anastasia Zueva.

“I didn’t swim at my full strength in the semi-final and after the semi, my coach and I studied video replays carefully and he asked to me pay more attention to details,” Zhao told reporters after she won gold in 59.05 seconds.

“The gold medal has significant meaning to me. It is definitely a huge boost to my confidence in the build-up to the London Olympic Games.”

Zueva, who won silver in both the 100 and 200 backstroke at the last world championships in Rome in 2009, was forced to settle for second best again, although she was desparately unlucky not to take gold, touching in 59.06.

“I am happy with the result, but I just lost by 0.01 sec,” Zueva said. “I made a mistake at the start.”

The 28-year-old Coughlin, who has now won 17 world championships medals in total, six of them gold, faded over the last 15 metres to take the bronze in 59.15.

“It’s my tendency to go out quick. The last 10 metres I felt very good,” said Coughlin, who took 18 months off from competitive swimming after the Beijing Olympics.

“It’s my best time since my break from Beijing and I am satisfied with today’s performance. Definitely I can go faster and next year will be quite promising.”