Iga Swiatek defeated world number one Aryna Sabalenka in the WTA Finals semi-final.
Swiatek's victory denied Sabalenka the chance to clinch the year-end top ranking.
Swiatek could be the year-end world number one by defeating Jessica Pegula in the championship match.
Jessica Pegula reached the WTA Finals championship match by defeating Coco Gauff.
The final was delayed to Monday due to rain and will occur in Cancun, Mexico.