The American told BBC Sport: "I'm excited. My goal is to win three games. That'll be my first goal. I'm going to go from there. Hopefully I can hold serve. That would be good. Most of all, I hope to break once.
"Caroline played a great match, it was really difficult today. I feel so bad. She's the only one undefeated [after the group stages] in this tournament and she deserved to win."
Halep is appearing in the end-of-season event for the first time, while Williams has won the competition four times, including for the last two years.