During the first morning of competition at the London Aquatics Centre, Michael Phelps, the two-time reigning champion in the 400-meter individual medley, advanced to Saturday night’s final — but just barely. And Tae-hwan Park of South Korea, the defending Olympic champion in the 400-meter freestyle, advanced in his event only after he won an appeal of a disqualification in his preliminary swim in the event.


Park was reinstated after a video review. The defending Olympic champion, he was told of his false start after winning his heat, the third of four, in 3 minutes 46.68 seconds, which would have been good for fourth place over all.


Park’s reinstatement knocked Ryan Cochrane of Canada out of the final. He had qualified eighth in 3:47.26. The Canadians were considering a protest, but it was unclear if they had any grounds to appeal.


Park looked shocked after the race and said that he did not know why he had been disqualified and that he would have to speak to his coach. A spokesman for FINA, the international governing body of the sport, said that it was the technical commission’s decision to overturn Park’s disqualification.


Park’s return to the eight-man field meant that his much-anticipated showdown with China’s Sun Yang was back on, as anticipated. Sun, the world-record holder in the 1500 meter freestyle, was the top qualifier, in 3:45.07. The two American entries, Peter Vanderkaay and Conor Dwyer, qualified second and third. The confusion surrounding Park was reminiscent of the men’s 200-meter backstroke final at the 2004 Olympics. The American Aaron Peirsol won with ease, only to be told he had been disqualified for an illegal freestyle kick at the 150-meter mark. Peirsol filed an appeal, his disqualification was overturned and he was awarded the first of consecutive gold medals in the event. In announcing its decision, FINA stated, “The disqualification of Aaron Peirsol was not accepted due to the detail of the reason supplied by the official being inadequate and not in the working language of FINA.”


Phelps, who is trying to become the first male to win a swimming event in three consecutive Olympics, came from behind on the final lap of his heat, the fourth of five, to touch out Laszlo Cseh of Hungary by one-tenth of a second. Phelps’s time of 4 minutes 13.30 seconds was the eighth-fastest of the morning. Cseh, the silver medalist four years ago and the bronze medalist in 2004, was ninth, which meant he would not be able to contend for a third consecutive podium finish.


“I would not have thought this would have been the result,” Cseh said.


“When I saw the time I was like, ‘Oh, it is going to be a challenge,’ ” said Phelps, who clocked a 4:07.82 in the heats four years ago to set up the first of his eight gold-medal swims in Beijing. He posted a 4:07.89 in the final at the United States trials last month.


“The only thing that matters is getting a spot,” said Phelps, who will be relegated to lane eight in the final, which will make it harder for him to see where he is in relation to his competitors in the middle lanes.


Phelps will be unable to track Lochte, who’s in Lane 3, which is not necessarily a bad thing. After the pair swam side by side in the 200-meter freestyle final at the U.S. trials, Phelps’ coach, Bob Bowman, said, “Ryan and Michael, when they’re next to each other, they are so focused on racing each other, they do stuff like not take it out so fast.” He added, “’Michael’s like ‘I’m ahead of Ryan, I'm okay.’And then Ryan is just waiting to make his move, and he makes his move and they do the cat-and-mouse stuff, and in the process of that they forget to swim fast.”


Phelps’s teammate and rival Ryan Lochte was the top seed in the 400 individual medley and led his heat comfortably until the last 25 meters, when he appeared to go on cruise control to conserve his energy and was overtaken by Chad le Clos of South Africa. Lochte, the reigning world champion, was timed in 4:12.35 to le Clos’s 4:12.24.


The top time was swum by Japan’s Kosuke Hagino, who posted a personal best of 4:10.01 while swimming in the heat immediately before Phelps. Speaking through an interpreter, Hagino said that he was pleased to finish ahead of Phelps and that his goal was to do it again in the final.


Lochte, the bronze medalist in the 400 individual medley in 2008, advised against counting out Phelps, who can become the first man to win a swimming event in three consecutive Olympics with a victory in the final. “He’s a racer,” he said. “We’re going to do everything we can to go 1-2 tonight.”