The result continues Stephens' poor run since her remarkable victory in New York in September, in which Stephens triumphed over Sevastova in the quarterfinals. The lost her opening matches in Wuhan and Beijing, she went down 7-5, 6-3 to Sevastova in Zhuhai.
Stephens, who will captain the U.S. in the Fed Cup final against Belarus, will get a chance to redeem herself when she faces Czech Republic's Barbora Strycova on Friday.
The 10th-seeded Rybarikova had lost her opening match to Julia Goerges in the Azalea group.
"I had a tough match yesterday and I had to play against someone who was resting so it was not easy for me," Rybarikova said. "I was saying to myself that I had to win this match because I had so many match points and it would be really cruel to end the season like this."
The tournament in Zhuhai features the leading players on the women's tour who didn't qualify for the WTA Finals in Singapore last week. The 12-player tournament is divided into four groups of three, with the winner of each group advancing to the semifinals.