Michael Phelps will swim in an outside lane in tonight's final of the 400 individual medley after qualifying for the eighth and final spot.
"A final spot is a final spot," two-time defending Olympic champion said, according to USA Today.
"The only thing that matters is getting a spot in. You can't win a gold medal from the morning."
However, he added: "I didn't expect those guys to go that fast in the heats."
His main rival, Ryan Lochte — whom Phelps beat at the 2008 Olympics in the 200- and 400-meter individual medleys — qualified in third place overall.
"It didn't feel so good," said Lochte, the bronze medalist in Beijing.
"But that was my first race and my first race never goes that well. I'm glad I got that out of the way."
He added: "It's hard... It's a tough field. But [Phelps is] in. You can't count him out."
Phelps won his heat in 4 minutes, 13.33 seconds, well off his world record of 4:03.84 set four years ago in Beijing, where Phelps won a record eight gold medals, the Associated Press wrote.
Lochte, who has won the 400 IM at the last two world championships, swam 4:12.35.
The New York Times wrote an in-depth piece about the rivalry between the two Americans, describing it as "sport’s version of a perfect wave: two fierce competitors, one with the winningest portfolio and the other with a winsome personality, fighting a turf war with the world bearing witness."
Japan's Kosuke Hagino, meantime, finished first overall in 4:10.01, a Japanese national record.
Were Hagino to finish first in the final, he would become the first Japanese swimmer to win an Olympic 400 IM gold medal.
Cyclists and swimmers took center stage at the London Olympics on Saturday, while badminton, archery, shooting, table tennis and judo events also started early on Day 1.
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