Elena Rybakina pulled out of her Pan Pacific Open semi-final on Saturday citing back problems, a day after securing her spot in the season-ending WTA Finals.
The 2022 Wimbledon champion was due to face Linda Noskova of the Czech Republic - who will now meet Sofia Kenin or Belinda Bencic in Sunday's final in Tokyo.
"I'm very sorry I can't play today," Rybakina, 26, said in a statement.
"I have been having problems with my back this week and can't play 100%. I am disappointed that my fans will not see me today but I hope to see you next year.