World number one Elena Rybakina withdrew from the Pan Pacific Open quarter-finals on Friday due to a neck injury, sending Canada's Leylah Fernandez through to the last four via walkover.
"Unfortunately, I’ve been experiencing inflammation and pain in my neck, which has been affecting my ability to play at my best," the Kazakh said in a statement on the WTA website.
Rybakina secured the year-end number one ranking after beating compatriot Yulia Putintseva in the second round.
Fernandez will face Russia's Kamilla Rakhimova for a place in the final.
Rybakina is expected to compete at next month's season-ending WTA Finals in Indian Wells, California.
