The Pole, who has played at every edition of the Finals since 2021 and won the event in 2023, had to reach at least the quarter-finals in Ningbo to have any chance of displacing Czech Karolina Muchova in the last qualifying place.
Swiatek was hampered by the injury in her 7-5, 6-1 loss to compatriot Maja Chwalinska in the Wuhan Open semi-finals last week. She has won only one tournament this year and failed to advance beyond the quarters at any of the four Grand Slams.
"After careful consideration, I've decided to withdraw from the tournament," the 25-year-old said in a statement posted on the Ningbo Open's social media.
"I've been experiencing an ongoing problem with my left thigh over the past weeks and I feel that giving myself the time to rest and recover is the right decision."
Muchova, who is not competing at the WTA 500 tournament in China, can still be overtaken by Marta Kostyuk. The Ukrainian has to make up 815 points at the Ningbo event and next week's Pan Pacific Open in Tokyo.
While Swiatek is unable to qualify directly for the November 8-15 WTA Finals in Indian Wells, California, she could go to the event as one of two alternates.