Australian Kay Kakakibara, a cyclist, entered a medically controlled coma after suffering head injuries during a BMX competition this week.
Sakakibara - a candidate to win an Olympic medal - fell in the first round qualifiers for a World Cup match in Australia last Saturday, and underwent medical treatment before being taken by plane to a hospital in the capital, Canberra.
The Sakakibara family said in a statement that the contestant underwent surgery on Sunday, and is expected to remain in a coma for the next two weeks.
"Kay's condition is critical, but stable," he added. "He is in good care 24 hours a day ... We realize that the path ahead is long and difficult ... We are positive and go about things day by day."
"At the moment we are waiting for how things will develop, and our focus is on treatment in the long run. Kay's career will stop with the BMX competitions this time."
Sakakibara was a candidate to win a medal, along with his sister Saya, when the Tokyo 2020 Olympics were held next summer.