Singapore's Joseph Schooling famously upset the great Michael Phelps to win Olympic gold but he said "my biggest rival is myself" as he bids to return to form at the Tokyo Games.

Schooling, the Southeast Asian city-state's first Olympic champion, said the year's pandemic delay had given him extra time to prepare after a disappointing world championships in 2019.

The 25-year-old caused a sensation at Rio 2016, when he beat his idol Phelps - who has a record 23 Olympic gold medals - by a fingertip to win the 100-meter butterfly.

Five years on, Schooling again looks an outsider and although he still wants to win "more than ever," he admitted much of his fate was beyond his control.

"It is always a never-ending pursuit of trying to get better. Win or lose, you're going to give it your best," he told AFP via email.

"In the past, I was always focused on winning, and I still am, I still want to win more than ever. But at the same time, it is also important to find a balance - the push and pull, letting go of what you can and can't control."

Schooling hasn't been at his best since Rio, and he crashed out in the heats of the 100-meter butterfly at the 2019 world championships in South Korea.

Later that year, he won only one individual title at the Southeast Asian Games in the Philippines, and was criticized by fans for being out of shape.

"I simply wasn't as prepared as I should have been for those two meets," Schooling said. 

"Despite the Tokyo Olympics being postponed, I think you can find positives out of every negative. 

"It gives me an extra year to get physically and mentally stronger, working on the things that can get me to where I want to be... I look at the extra year as a positive boost for myself."