London - South Korean Park Tae-Hwan's hopes of defending his Olympic title lay in ruins after his sensational disqualification from the 400-metres freestyle on the opening day of the swimming competition here Saturday.
Gold medal favourite Park, who holds the Olympic and world crowns in the event, touched first in his heat but was disqualified for a false start, slamming the door on his chances of defending the title he won at the 2008 Beijing Olympics.
Chinese rival Sun Yang cashed in on Park's shock exit by topping the qualifiers into the final later Saturday with a best time of three minutes 45.07 seconds.
Park had come through to win his heat in 3:46.68, but the crowd gasped when the semaphore board flashed that the Korean had been disqualified.
"I don't know what happened," a bewildered Park told reporters shortly after leaving the pool. "I need to speak to my coach to find out."
Park, a two-time 400m world champion, who spent time training in Australia as preparation for the London Olympics, has never been beaten by Sun in international competition.
His demise dramatically changed the gold medal reckoning with Sun, the 1500m freestyle world record holder, now the favourite to go after the 400m freestyle Olympic gold medal ahead of Americans Peter Vanderkaay (3:45.80) and Conor Dwyer (3:46.24).
Sun owns the fastest time in the world this year of 3:42.31, and his personal best of 3:40.29 makes him the third-fastest performer in history behind Paul Biedermann and Ian Thorpe.
Park became a high-profile casualty of the cut and thrust of qualifying for an Olympic final.
"That's the Olympics. It's always a surprise, every single heat. You just have to focus on your own race," said Canadian Ryan Cochrane, who squeezed into the eight-man final after winning the second of four heats.
"It's super hard to know what your competitors are going to do when you're in the first seeded heat and you haven't seen any other heats," Cochrane said.
While Park was coming to terms with his dramatic early exit there was also disappointment for German world record holder Biedermann, who also missed out on the final with his 12th-fastest time of 3:48.50.
"It wasn't so good," Biedermann said. "I wanted to lead from the front, but I couldn't hold it.
"I have the 200m freestyle tomorrow and I am hoping for a better outcome."