Answer
Kontaveit retired from tennis at the age of 24 due to a degenerative condition of the lumbar discs in her spine. The lumbar spine is the lower back, and the discs are the cushions between the vertebrae. Degeneration of the discs can cause pain, stiffness, and difficulty with movement. In Kontaveit's case, the degeneration was severe enough to make it impossible for her to continue playing tennis.