The American confessed that she felt for her opponent after the game, adding that it is Wozniacki who deserves to be in the final.
"Caroline played a great match, it was really difficult today. I feel so bad," BBC Sport quotes the 33-year-old as saying.
"She's the only one undefeated [after the group stages] in this tournament and she deserved to win."
Williams will take on Simona Halep in the final in Singapore.