Korean star Park Tae Hwan has opened up a telling crack in Grant Hackett's armoury by stealing his 400m freestyle crown at the world swimming titles.

The Queenslander just couldn't keep up the pace in the final lap as 17-year-old Park (3:44.30) and Tunisia's Oussama Mellouli (3:44.82) finished over the top of him.

Having drawn lane eight after only qualifying for the final by 0.09s, the most decorated swimmer in world championships history was always up against it.

Hackett threatened to pull off victory by staying near the front until the final lap, fading to collect bronze in 3:45.43 - nearly three seconds outside his best time.

"It's really difficult outside the pool to see across and see everyone's position," Hackett said afterwards.

"So I just had to take it out, swim my own race and hope I touched the wall in first place but unfortunately I didn't.

"But top three is still a good result. I felt fairly awful this morning so it's always nice to get on the medal dais still."

The result ended Australia's domination in the event, having won the eight-lap race at every world championships since 1991.

The result will give Hackett's rivals plenty of heart before lining up against him in the 800m and 1500m later in the week.

Hackett was the swimmer of the meet at the 2005 Montreal world championships but little has gone to plan since then.

His body started to rebel due to the arduous demands of his craft and he missed last year's Commonwealth Games after booking himself in for shoulder surgery.

The long road back has had its fair share of potholes, a shortened taper exposing him at the national trials before a sudden move from the Gold Coast to Melbourne last month to be with his fiancee.

He is getting married next month and said the move south was pivotal to him being ready for the 2008 Beijing Olympics.

The move split up his successful long-term partnership with Denis Cotterell.

The question must be asked if the fire still burns as it once did for the endurance machine.

Hackett said he wasn't surprised by Park's performance after the teenager stormed home on the final lap.

"I'm not surprised to see him dropping those times. He's been improving, he's worked very hard and he's very focused," he said.

"He's been spoken about as being one of the main rivals next year for Beijing, but I certainly think obviously he was a force to be reckoned with tonight and throughout the rest of these championships."

Hackett had said before the meet his disrupted preparation had left him vulnerable and his opening day's performance demonstrated that.

"I just had to go out there and give it my best shot and was happy I was at the pointy end of the field but just not the gold medal unfortunately," he said.

"But realistically I would have given myself more chance if I was right next to Park but it was not to be tonight.

"But it is the best time I have done since before my shoulder operation, so I have to be happy with the time for now and use it as motivation."

The result has meant that Hackett's dominance in the 800m and 1500m is under threat.

"No doubt it means that it is going to be very competitive in the 800 and 1500 with Park in the 15, he is swimming very well, he is focused and has prepared very well for this meet," Hackett said.

"At the same time I was swimming quite well myself. I was right there with him. I certainly think for me that puts a bit of confidence in me."

Park said through an interpreter he never intended to go out as quickly as Hackett.

"I wasn't going to go at the pace of Hackett," Park said.

"Then after 200m I started to push and went even harder in the last 50m. I am mentally really tired but confident after I've got the gold medal."

Park burst on to the world stage last year when he won the 400m-1500m freestyle double at the Pan pacific championships in Canada then seven medals, including three golds, at the Asian Games in Qatar.

He joined Japanese breaststroker Kosuke Kitajima as the only Asian men to win world titles.

"When I saw my name on the screen I forgot how I was feeling," Park said.

"I am the first Korean world champion in (swimming) history.

In the women's 400m freestyle, Frenchwoman Laure Manaudou threatened her world record before fading to win in 4:02.61 - just half a second outside her world record.

Reigning title holder Jess Schipper (57.57s) warmed up for Tuesday night's 100m butterfly final by finishing the fastest qualifier ahead of compatriot Libby Lenton (57.78).

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