Defending champion Iga Swiatek thrashed Russia's Daria Kasatkina 6-1 6-0 at the WTA Finals - but faces an anxious wait to see if she will qualify for the semi-finals.
Polish second seed Swiatek dismantled Kasatkina - an alternate who replaced the injured Jessica Pegula on Wednesday - in only 51 minutes at the end-of-season event in Riyadh, Saudi Arabia.
However, Swiatek will be knocked out if Wimbledon champion Barbora Krejcikova beats Coco Gauff, who has already qualified, later on Thursday to reach the last four.
Five-time major champion Swiatek, recently deposed as world number one by Aryna Sabalenka, beat Krejcikova and lost to Gauff in her first two group matches.