Sun Yang won his first gold medal of the 2013 World Swimming Championships when he claimed the gold medal in the men's 400 meter freestyle on Sunday. [CFP]

Sun Yang won his first gold medal of the 2013 World Swimming Championships when he claimed the gold medal in the men's 400 meter freestyle on Sunday.

The 21-year-old repeated his feat from the 2012 London Olympic Games with a commanding swim, which saw him finish in an impressive time of 3:41.59, almost two and a half seconds ahead of Japan's Kosuke Hagina, who made it an Asian 1-2 in the event with a time of 3:44.82. American Conner Jaeger was just three hundredths of a second back to the Japanese but edging Ryan Cochrane, who had been second fastest in qualifying, into fourth place.

"I think this 400 meters win is important. I achieved my goal tonight and I proved myself after so many problems, so I am very satisfied with the result," said Sun.

As in the morning semi-final Sun Yang was quickly into his rhythm and opened up a comfortable lead, moving at a pace slightly below that of Paul Biederman who claimed the world best time in his World Championship swim four years ago in Rome.

Perhaps mindful of the fact he has heats in the 800 meter freestyle on Tuesday as well as the 1,500 meters on Saturday, Sun Yang was content to maintain a pace which kept him around a body length ahead of his rivals.

"I hope to be able to continue with my good performances in the coming days," he added, before speaking of the importance of the medal for him.

"I win medals for my coach and for my country. I want to thank my ex-coach and my current coach," said Sun Yang.

It was a brilliant way for him to return to form after suffering a shoulder injury over the summer and means the Chinese swimmer has got off to a perfect start in his quest for a World Championship treble. He admitted he had cried after winning the race as he had remembered having to train alone after a falling out with coach.

"Training alone was an unforgettable situation but my problem has not been the coach, my problem is to get back to my best form. I have trained hard every hour of every day and I want to get better results," he said.

Double Olympic gold winner, Ye Shiwen was very comfortable in qualifying for the final of the women's 200 meter medley, going through in the second fastest time of 2:09.12, just over half a second behind Hungarian, Kainka Hosszu.

America's Katie Ledecky claimed the second World Championship gold medal to be decided in the pool with a stunning sub 4 minute swim on her way to the women's 400 meter title.

The 16-year-old showed complete control as she stormed ahead of the field finish 2.62 seconds ahead of second placed Spaniard, Melanie Costa-Schimd, who clocked a time of 4:02.47 and New Zealand's Lauren Boyle who claimed bronze in a time of 4:03.89.

"This feels like a gold medal for me as it has been so difficult," said the Spaniard. "I never thought it would happen. I knew Katie would start quickly, so I just tried to follow her and try and try and do my best."

There was also joy for the Americans in the women's 4x100 meters freestyle relay as the team led by Missy Franklin and Natalie Coughlan beat traditional rivals Australia into second place to claim gold.

The American quarter finished in 3:32.31, just 12 hundredths of a second ahead of the Australians to give Franklin, who was one of the sensations of the 2012 Olympics, her first gold medal of the Championships, although it was Megan Romano who spoke for the team.

"The Australians started strongly, but all of our team was strong and that made it easy for me on the anchor leg," she commented, modestly highlighting the fact that the entire team had swum fast times, without mentioning her anchor leg of 52.60 had taken them to a new American record.

However, American hopes of a gold winning treble in the first day in the pool came to a spectacular end when the French came from behind to take gold in the 4x100 men's freestyle relay.

The Americans had looked to be on their way to gold as Nathan Adrian and Ryan Lochte swam solid opening legs to give them a half second lead in the race before Fabien Gilot got the French into contention with a third leg of 46.90 seconds.

Jeremy Stravius then swam a magnificent anchor leg to come from behind and give the French gold in a race which had looked to belong to the American's after the first two legs.

"Everything happened very fast as we came back, but we are all very happy," said Gilot.

"The French relay team now has something beautiful to live again. It was a big challenge to succeed in this race, but our strategy paid off and we succeeded," he said, delighted at the support the team had received from the numerous French supporters in the Barcelona pool

"I felt like we were at home," he said, while the American's were clearly shocked to have failed to win the World Championship gold for the first time in 10 years and admitted they had not expected the French to challenge.

The Russian quartet also produced an exceptional swim which saw them finish just 0.2 second behind the Americans to take the bronze medal ahead of the Australian team, which disappointed as it had done in London 12 months earlier.

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