James Guy finished sixth in the men's 400m freestyle final, as Sun Yang of China won Gold by over two seconds in at the FINA World Aquatics Championships in Budapest, Hungary.
Australia's Mack Horton took the silver, with Italian Gabriele Detti finishing in third, comfortably behind the Chinese.
After a strong start from the Briton, the 21 year old from Bury was left in the waves of his opponents and faded badly over the last 100m in sixth place.
Early battle for position
The opening lengths were keenly-fought battle for early positioning, with James Guy, Sun Yang and Park Tae hwan exchanging the lead over the opening four lengths of the Duna Arena pool.
With Sun more known for his strong finish over the distance, the Chinese opted to break away early at the halfway stage and opened up a significant lead.
With both Mack Horton and Gabriele Detti closing at the top of the pool in Lanes 1 and 2 however, the race was still up for grabs.
Sun cruises to gold
As Australian Horton began to chase the reigning Olympic silver-medalist down, Sun merely opened up a greater lead in what quickly became a one-horse race, further extending his lead over the rest of the field.
Guy meanwhile was clinging onto the coat-tails of Detti in Lane 7, but was being slowed by the chopping waves of his opponents ahead.
Turning for home with over a second advantage, Sun was on world record pace, with even a sub 3 minutes 40 seconds on the cards. The Chinese eased down over the final strokes however, to claim his first Gold of the Championships by over two seconds.
James Guy will now switch attentions to his stronger event of the 200m free - where he defends his world title - on Tuesday.