Kosuke Hagino won't be taking part in April's Japanese national swimming championships.
The Rio 2016 gold medallist in the 400m individual medley cited a lack of motivation for his decision, after an extended period of poor form which has seen him produce underwhelming results.
In a statement, he said: "I’ve been unable to achieve the results I had hoped for since the 2017 season. As the gap between my targets and reality has widened, it has become harder for me to maintain my motivation."
The decision has caused concern in Japan, with two of the country's top medal hopes now doubts for their home Games at Tokyo 2020. Rikako Ikee, Japan swimming's poster girl, is currently fighting leukaemia.
Hagino won two medals at Rio 2016 — finishing second behind Michael Phelps in the 200m medley five days after winning the 400m — but has been unable to hit top form since.
The 24-year-old has not achieved an individual win internationally since the Olympics, with the exception of a short-course 400m IM during the 2018 World Cup. He was, however, part of Japan's 4×200m freestyle relay team that won gold at last year's Asian Games.
Last month, the four-time Olympic medallist withdrew from a high-altitude national training camp held in Spain.
"I have accepted the fact that I am in no condition to focus on the sport, and thus I have made the decision," Hagino added in his statement.
"I’m still not mentally ready to compete at full strength."
Gold for Japan in Men's 400 IM
Gold for Japan in Men's 400 IMKosuke Hagino catches up on the freestyle leg and beats American Chase Kalisz in the last 25m of the men's 400m individual medley final.
World Championship doubt
Hagino's withdrawal from the national championships means his place at this year's World Championships, to be held in Gwangju, South Korea, in July, are in doubt.
The Japanese swimming federation use the nationals as a qualifying event to select their team for the global spectacle.
National coach Norimasa Hirai has acknowledged the challenge facing Hagino.
"It’s obviously not a plus that he’s not entered the nationals. It’s a critical situation for him but he has to stay positive," he told Japanese media.
"He’s not in very good spirits. He needs to rest and when he decides to comes back, he needs to start swimming for himself.
"The ball is in his court."