Larsen Jensen improved on his own American record and won Saturday's 400-meter freestyle heats in 3 minutes, 43.10 seconds for the best time of the year.
Local favorite Zhang Lin of China produced a roar inside the Water Cube when he beat world champion Park in his heat to qualify second in 3:43.32.
Park Tae-hwan the South Korean who upset Hackett on the Australian's home turf at last year's worlds, had to settle for qualifying third in 3:43.35, while Grant Hackett found himself down in fifth with 3:44.03, despite swimming the best heat of his career at the distance.
Russian outsider Nikita Llobintsev touched fourth in 3:43.45.
"Look how close it is. It's anybody's at this stage," Hackett said. "It's just a matter of doing everything right between now and (Sunday's final)."
Hackett, whose specialty is the marathon-like 1,500 free, is attempting to give Australia its third consecutive Olympic victory in the 400. Countryman Ian Thorpe won it in 2000 and 2004, but has since retired.
"I've done the work and I'll do the best I can to compete for my country tomorrow," Hackett said, adding that he was a bit surprised by the speed in the heats. "I knew it would be quick, but not quite that quick. I didn't think 43.1 would be the fastest. I thought maybe 44 or 43.8. Everyone had to work hard, that's for sure."
If the heats were any indication, Thorpe's world record of 3:40.08 _ set in Manchester, England, in 2002 _ could come under pressure in the final.
"You have to go under 41 to win," said Yuri Prilukov, the Russian who took the eighth and final qualifying spot in 3:44.82.
Prilukov is one of the few top competitors still using an Arena swimsuit. Speedo's LZR Racer is the current suit of choice among Olympians, and has been worn for 47 of the 51 world records set this year.
While many swimmers insist that the LZR can help gain half a second per 100 meters, Prilukov isn't a believer.
"It's a good suit," Prilukov said, pointing to his Arena tag. "I'm going to swim tomorrow with it."
Later on at these games, Prilukov, Jensen and Hackett will battle again in the 1,500, with Hackett attempting an unprecedented third straight victory.
