Relief was etched all across Michael Phelps' face on Wednesday after the American won his first gold medal of the world swimming championships in Shanghai, one year to the day before he begins his final Olympics campaign.

Phelps, the most successful Olympian of all time who won an unprecedented eight golds in Beijing in 2008, has said he will retire after the London Olympics that open on July 27 next year.

A bronze in the men's 4x100 freestyle relay and silver behind compatriot Ryan Lochte in the 200 freestyle as well as recent defeats in the 200 butterfly -- his signature event -- in the lead up to Shanghai had left the 26-year-old facing 12 months of gruelling preparation under coach Bob Bowman.

His victory in the 200 butterfly on Wednesday, however, at least indicated he has a base to build on his way back to his best form.

"I'm happy but this is just a small step for my next year and ... I'm super happy with swimming faster than I did than last year ... but I still want more and I want to be faster," Phelps said.

"Being able to swing all these semis and finals, I haven't been able to do this (physically) for a while.

"So (I'm) trying to see what I can do to get through a week like this, to see what kind of shape I need to be in is something we wanted to do coming in and that's something we are finding out."

To top off victory, Phelps also qualified for Thursday's 200 individual medley final with Lochte, the 2009 world champion, after the pair swam the two fastest times.

Chinese teenager Sun Yang whipped the home crowd into a patriotic fervour as he controlled the 800 freestyle final from start to finish to win China's third gold medal in the pool, and 13th overall in Shanghai.

It was also just the second gold won by a Chinese man in swimming at the championships since their inception in 1973, though their women have claimed 29 world titles.

"This is my first world championship win (and) ... I am very pleased with the gold medal," the 19-year-old said after he had thrown his victory bouquet into the crowd and then handed his commemorative soft toy mascot to a young girl. "It was a smooth swim."

Italy's Federica Pellegrini won the women's 200 freestyle to secure the 22-year-old an unprecedented double-double, having won both the 200 and 400 freestyle at the same championships twice.

The 200-400 double had only been achieved on four previous occasions, though Pellegrini is the first to do it twice.

"I'm so thrilled today and very pleased with my performance so far. The gold medal shows that I have consolidated my status in freestyle since (the) Rome world championships," Pellegrini said.

Brazil's Felipe Franca Da Silva claimed Brazil's second gold in the pool with victory in the men's 50m breaststroke.

Australia's James Magnussen produced an incredible burst of speed over the second lap of the 100m freestyle to be the only man under 48 seconds in the semi-finals and qualify fastest for Thursday's blue riband final.

"I think this is a very important meet, because I will be dealing with the same people in the Olympics," said the 20-year-old Magnussen, who is competing at his first world championships. "I am a confident swimmer. When I am confident crazy things happen."