Marchand won four individual gold medals at the Paris Olympics in 2024 but his efforts at his home Games left him exhausted and needing time to rest.
He returned to action in Florida in May and arrived in Singapore with only the 200 and 400 medleys on his programme for individual events.
The 23-year-old has slipped back into competition mode with ease and produced a stunning 1:52.69 in his 200 medley semi-final to shave nearly one-and-a-half seconds off Ryan Lochte's mark (1:54.00) set at the world championships in Shanghai in 2011.
The Toulouse native shook his fist and punched the water in celebration, and later said the decision to come in with a lighter workload had been vindicated.
"It was probably the right decision," Marchand said.
"I'm grateful for my coaches, all the staff behind me. It's been a pretty hard season for me, but I’m so happy to be here."
Marchand, who also holds the 400 IM world record, said he needed a moment to process his achievement on Wednesday before finally finding the words to describe his feelings.
"It's just an explosion of joy," he added.
"I feel all the choices I made this year were the right ones and I want to thank everyone who's been supporting me. Since the Games, I've had this goal in mind. I didn't expect things to come together again so quickly.
"Right now, I'm trying to get some good sleep, which isn't easy, but tomorrow's the final, and it's going to be great."