Leon Marchand admitted he was "struggling" on the final day of the world championships on Sunday after qualifying for the 400m individual medley final with an uncharacteristically sluggish swim.
The French superstar broke the world record and won gold in the 200m individual medley earlier this week in Singapore but he was seventh fastest in the 400m medley heats with a time of 4min 13.19sec.
Marchand, who broke Michael Phelps's 400m individual medley world record at the world championships in Japan two years ago, said his performance "worries" him ahead of Sunday night's final.
"I would have preferred to set the best time this morning and it didn't go as planned, I was struggling a bit," said the 23-year-old.
"Strange, because I slept well, I felt fine this morning, but that's swimming – sometimes you dive in and you don't feel good."
Japan's Tomoyuki Matsushita set the fastest time in 4:10.39, almost three seconds quicker than Marchand.
Marchand is swimming a lighter programme than usual in Singapore, competing only in the individual medley and relay events.
His heat on Sunday morning was only his fifth swim of the championships.
"It worries me, but I have a lane tonight so in the end it's OK," said the Frenchman.
Summer McIntosh was fastest in the women's 400m individual medley heats, the morning after her epic battle with American great Katie Ledecky in the 800m freestyle final.
McIntosh saw her bid to join Phelps as the only swimmers to win five individual golds at a single world championships end after finishing third behind Ledecky and Australia's Lani Pallister.
McIntosh said she "got all of my thoughts and emotions out last night", before coming through the 400m individual medley heats in 4:35.56.
"I'm really just focusing on the 400 IM and putting a good time down and touching the wall first," said the 18-year-old Canadian, who has already won three golds in Singapore.
"There's more reflection that I can do from what happened last night in the 800 but I think that's reflection that will be happening at the start of next season."
Chinese 12-year-old Yu Zidi also qualified for the final in a third-fastest time of 4:36.49, less than a second behind McIntosh.
Yu won her heat and will be appearing in her third individual final in Singapore.
She finished fourth in both the 200m individual medley and 200m butterfly and picked up a bronze medal as a member of China's 4x200m women's freestyle relay team.
Yu became the youngest swimmer in history to win a world championships medal.