Michael Phelps and Grant Hackett, two men who have taken completely different paths to swimming greatness, are suddenly poised to collide when the 11th world championships get underway in Montreal on Sunday.

The American Phelps and Hackett of Australia are already assured of their places in their hall of swimming greats but their quest for sporting immortality has brought them face to face in the most eagerly awaited contest of the championships.

Not content with winning six gold medals at last year's Athens Olympics [Images], Phelps is chasing a record eight titles in Montreal, two more than Ian Thorpe's world title haul in Japan [Images] four years ago and one more than Mark Spitz's legendary haul from the 1972 Munich Olympics.

Hackett, the undisputed king of long-distance swimming, is chasing his own slice of history, bidding to become the first swimmer to win the same event at four world championships.

He is also chasing an unprecedented four freestyle titles from 200m to 1500m as well as striving to overtake Thorpe and Jenny Thompson's record of 13 career medals at the world championships. He currently has 12.

Both men are almost certain to walk away with a stack of medals in Montreal but their individual clashes in the 200 metres and 400m freestyle will determine how many of them have that golden glitter.

Phelps gave up the chance of almost certain victories in two of his favourite events for the opportunity to challenge Hackett in the 400m.

The Australian, runner-up to Thorpe in the eight-lap event at the last three world championships, returned the compliment by agreeing to face Phelps in the 200m, an event the American is favourite to win.

"The statement, 'The sky is the limit', really is true," Phelps said.

"The 400m is going to be a tough race. Hackett has been very, very dominant, very strong. It's my job to step up and try to race him. I'll try to go out after it and see what happens."

Phelps and Hackett won't have to wait long for their clash. The 400m, scheduled for Sunday night, is the first final of the eight-day, 40-event swimming programme.

INTENSE RIVALRY

Their clash has not only added a sense of drama to a championship diluted by the absence of so many reigning Olympic gold medallists -- including Thorpe, Pieter van den Hoogenband, Inge de Bruijn, Gary Hall, Yana Klochkova and Amanda Beard -- but has also increased the intense rivalry between the Australian and United States teams.

The competition between swimming's two traditional superpowers has never been greater and when the championships end, the two countries will stage their own "duel in the pool in California".

The U.S. hold a distinct advantage in the men's events with Aaron Peirsol holding the world record for the 100m and 200m backstroke, Brendan Hanson the world record in the 100m and 200m breaststroke and Ian Crocker the fastest ever 100m butterfly.

The Americans are also strongly favoured to win the three men's relays in the absence of the South African 4x100m freestyle quartet that won gold in Athens but the Australians hold most of the aces in the women's races.

Australia boasts the fastest three freestyle sprinters in the world, led by triple Olympic gold medallist and world record holder Jodie Henry.

They are also favourites to win at least two of the three relays while Leisel Jones is on course to end her run of near misses in major events by winning gold in the breaststroke.

The American challenge will rest heavily on the performances of the versatile Natalie Coughlin and the rapidly improving Katie Hoff, while Europe is also expected to figure highly in the women's races through Polish butterflyer Otylia Jedrzejczak and French freestyler Laure Manaudou.

Hackett concedes his team has no real hope of beating the Americans on the medals table but has warned his great rivals not to underestimate the Aussies.

"Realistically, we're not at their standard, but it's always nice to go in as an underdog and be competitive against them," Hackett recently told reporters.

"This is an opportunity for a lot of people who have been perhaps just outside of a final, or just outside of a medal place, to be able to step into the limelight."