The crowd at London's Aquatic Centre roared with delight when the defending champion won her heat in a time of four minutes and 05.75 seconds.
But the cheers soon turned to anxiety as the swimmers in the following two seeded heats began posting faster times. Then there was relief when Adlington made it through to the final as the eighth and slowest qualifier.
"It felt faster," Adlington said. "You just don't know being in the first (seeded) heat. I had no option, I just had to go for it."
Italy's Federica Pellegrini, the reigning world champion and world record holder, also flirted with danger but finished seventh overall.
"The race is tonight. It's not as if I've had great results in the morning this year. Now we'll see this evening," she said.
France's Camille Muffatt set the fastest time of 4:03.29, just ahead of American Allison Schmitt, who trains in Baltimore with Michael Phelps, who bombed out in the 400m individual medley on Saturday.
"I am excited to have a lane for tonight. It's a fast meet all around," Schmitt said.