The four-time Grand Slam winner topped the rankings for 99 weeks before ceding her position to Kazakhstan's Elena Rybakina, who also beat her in the US Open final this month.
It means Sabalenka has not won a title since March and will end the year without a major for the first time since 2022.
"This time I feel like, because I didn't play that many matches, I feel fresh," the 28-year-old Belarusian told reporters ahead of the China Open in Beijing.
Despite a lacklustre few months by her standards, her New York showing was "the moment when I started to feel healthier and stronger", she said.
"The middle part of the season... didn't go that well. I struggled a little bit with different stuff.
"At the US Open I started to enjoy tennis again because of being healthy mentally and physically."
Sabalenka, who withdrew from Beijing last year due to injury, begins her bid for a first title there in the round of 64, against either Hungary's Anna Bondar or Mexico's Renata Zarazua.
She is seeded second behind Rybakina, who faces either Yulia Starodubtseva of Ukraine or Alina Charaeva of Armenia.
Last year's champion Amanda Anisimova will not be taking part, having announced on social media that she was ending her season early over a wrist injury.
In the men's draw, fan favourite Novak Djokovic is looking to add to his six titles at the China Open, where he has never lost a singles match.
"My record speaks for itself on Chinese soil. Knowing that, it also brings this dimension of confidence that I really need," the Serbian, 39, told reporters on Monday.
"Overall it's been really good so far with the body, the way I'm feeling and playing."
Djokovic has won a record 24 Grand Slam titles but is currently ranked 11th in the world. He begins his Beijing campaign against Portugal's Nuno Borges in the first round.
