Michael Phelps scraped into a 400m medley showdown with Ryan Lochte while South Korean hero Park Tae-Hwan was disqualified then reinstated on a turbulent opening day of the Olympic swimming competition here Saturday.
Park, whose 400m freestyle victory in Beijing was South Korea's first Olympic swimming gold, touched first in his heat but was stunned to look up and see the scoreboard showing he was disqualified.
A South Korean appeal was rejected by the competition referee.
But world governing body FINA's jury of appeal overturned the disqualification on the advice of their own technical commission, although a FINA spokesman declined to elaborate on the reasons for the reversal.
Park's time of 3:46.68 made him the fourth-fastest in the heats led by Chinese rival Sun Yang.
His appearance in the final later Saturday will leave Canada's Ryan Cochrane, who thought he had secured the eighth and last spot for the medal race, out in the cold, but Canadian team officials opted not to protest.
Park's rollercoaster day followed drama for 14-time Olympic gold medallist Phelps, who won his 400m medley heat only to find his leisurely time of 4min 13.33sec put him into the final by the skin of his teeth in eighth place.
Hungarian Laszlo Cseh, who earned silver behind Phelps in Beijing and bronze in Athens, was second in that heat and didn't make the final, in which Kosuke Hagino led the way.
Cseh's exit was just one of several unwelcome surprises for some of the sport's marquee names on the opening day at the Aquatics Centre, where Britain's Queen Elizabeth II made a brief appearance and US presidential candidate Mitt Romney was among the spectators.
Paul Biedermann of Germany, who set the 400m free world record at the height of the high-tech bodysuit era in 2009, failed to make the final, clocking the 12th-fastest time of the morning as Sun shone with a time of 3:45.07.
Phelps insisted his lackluster qualifying performance didn't rule him out of gold medal contention.
"I think the only thing that matters is really getting a spot in," he said. "You can't win the gold medal from the morning."
Lochte said he'd certainly still be keeping an eye on his compatriot.
"It's a tough competition, he's in the final and you can't count him out," Lochte said, adding that the heats in the grueling four-stroke event were all about getting into the final.
Lochte was content to finish second in his heat behind South African Chad le Clos, whose 4:12.24 was second-fastest of the morning. Lochte was third-fastest overall in 4:12.35.
Meanwhile, Hagino was delighted to set a Japanese record.
"I already broke the national record in April, so I just beat my personal best," he said.
Phelps isn't trying to match his great haul of China -- eight gold medals in eight events -- but he does have a chance to make more Olympic history.
As the two-time defending champion in all four of his individual events -- the 100m and 200m butterfly and 200m and 400m individual medley he could become the first male swimmer to win the same Olympic event at three successive Games.
If he doesn't work some magic in the 400m medley final on Saturday, Japan's Kosuke Kitajima could be the first male swimmer to do it.
Kitajima, the two-time defending champion in both the 100m and 200m breaststroke, made it safely into the semi-finals of the 100m breaststroke with the second-fastest time -- just one-hundredth of a second behind Australian Christian Sprenger.
American world champion Dana Vollmer led the way into the semi-finals of the 100m butterfly in an Olympic record of 56.25sec.
Aussie great Stephanie Rice, nursing a shoulder injury heading into her defence of the 400m medley title she won in Beijing, booked her berth in the final with the seventh-fastest time.
American Elizabeth Beisel led the way into the final, improving on her own world leading time of the season in 4:31.68. China's Ye Shiwen was second-fastest in an impressive 4:31.73.
The three medal-winning teams from Beijing led the way into the final of the women's 4x100m free relay, but it was 2008 bronze medallists Australia in front of the United States and reigning champions the Netherlands -- who rested their rising freestyle sprint star Ranomi Kromowidjojo in the heats.