Then she remembered it's about bouncing back.
Manuel did just that in the 100 freestyle, winning her second straight title at the world championships on Friday.
Relegated to Lane 1 with the slowest qualifying time, the American led all the way and touched first in 52.04 seconds, beating Cate Campbell of Australia by 0.39 seconds.
"Not everybody has a perfect swim every time so I just needed to regroup and put on a good face," the 22-year-old Texan said. "I'm a veteran on the team, so I have to be able to show a little bit of poise in these hard moments."
Manuel was unable to hold off Campbell on the anchor leg of the 4x100 free relay earlier in the meet, with Australia claiming gold and the U.S. silver.
"I did take that relay really hard because I didn't feel like I did as best as I possibly could," Manuel said.
She made up for it in the 100 free.
Manuel was the only woman under 25 seconds on the opening lap. She knocked 1.27 seconds off her time from the morning semifinals that landed her in the far outside lane.
Manuel became the second woman to win multiple titles in the 100 free, joining Kornelia Ender of the former East Germany, who won in 1973 and '75.
Sarah Sjostrom of Sweden, the world-record holder, took bronze.
In 2016, Manuel became the first African-American woman to win an individual swimming gold at the Olympics when she tied Canada's Penny Oleksiak for gold in Rio de Janeiro.
Mack Horton rallied Australia to victory in the men's 4x200 freestyle relay, putting them in front with a lap to go. Clyde Lewis, Kyle Chalmers, Alexander Graham and Horton won in 7:00.85.
Russia took silver in 7:01.81. The U.S. team of Andrew Seliskar, Blake Pieroni, Zach Apple and Townley Haas earned bronze in 7:01.98.
Sun Yang swam China's anchor leg against Horton, but he wasn't able to move up and his team finished sixth in his final event.
Sun swept the 200 and 400 freestyles, winning the 200 via disqualification and beating Horton in the 400. Horton snubbed Sun on the podium because the Aussie is angry that FINA allowed him to compete this week while facing a career-threatening hearing involving a clash with drug testers.
Leading off the relay for Britain was Duncan Scott, who snubbed Sun on the podium after the 200 free for similar reasons as Horton.
Yuliya Efimova of Russia claimed a record third title in the 200 breaststroke. She won in 2:20.17 without competition from American rival Lilly King, who was disqualified in the preliminary heats when she didn't touch both hands on the wall at the same time in a turn.
"I'm sad that she's not here because it would probably mean a more interesting race," Efimova said. "I told my dad after semifinals that I swam fast and I don't know what to do in finals because I did everything already."
It was Efimova's 14th individual world medal, tying her with Katinka Hosszu of Hungary and Sjostrom for most among women.
Tatjana Schoenmaker of South Africa earned silver, becoming her country's first woman to win a swimming medal at worlds. She was in tears on the podium and received her medal from countrywoman Penny Heyns, who swept the breaststroke events at the 1996 Atlanta Olympics.
"I was completely shocked," Schoenmaker said.
Sydney Pickrem of Canada took bronze.
Anton Chupkov of Russia broke the world record in the 200 breaststroke, touching in 2:06.12 in defense of his title from two years ago in Budapest.
That lowered the mark of 2:06.67 that Matthew Wilson of Australia had tied in the semifinals a day earlier. Ippei Watanabe of Japan, who earned bronze, set the mark in January 2017 in Tokyo.
Chupkov was eighth after the first lap and moved up to fifth at 150 meters. He came home in 31.89 to overtake Wilson, who led from the start and finished second.
Evgeny Rylov of Russia defended his 200 backstroke title, beating Olympic champion Ryan Murphy after the two dueled through the closing meters.
Rylov led all the way and touched in 1:53.40. Murphy settled for silver in 1:54.12. Luke Greenbank of Britain earned bronze.
Two more world records fell in semifinals at the hands of Americans.
Caeleb Dressel broke Michael Phelps' record in the 100 butterfly and Regan Smith lowered the mark in the 200 backstroke.
Dressel won his heat in 49.50 seconds — 0.32 seconds better than Phelps' mark set at the 2009 world meet in Rome at the height of the high-tech suit era.
"Just the standard that Michael set, wanted to go after it," Dressel said. "I hope he was happy watching me do that."
Dressel was out in 22.83 seconds — 0.53 seconds under Phelps' pace — and came home in 26.67 to lead eight men into Saturday's final.
Smith, who is 17, won her semifinal heat in 2:3.35. That erased the old mark of 2:04.06 set by Missy Franklin at the 2012 London Olympics.
"I'm in shock," Smith said. "I really don't believe it."
Dressel was impressed with his young teammate.
"I thought that was one of the top-three toughest world records on the women's side and she just crushed it," he said.
Franklin tweeted: "You're beyond a joy to watch."