The defending champion, Iga Swiatek, was knocked out of the WTA Finals after Barbora Krejcikova defeated Coco Gauff to book her spot in the last four.
Swiatek had earlier needed less than an hour to dispatch Daria Kasatkina, who had replaced the injured Jessica Pegula, 6-1, 6-0 in the opening match of the day in Riyadh.
But her chances of progress depended on the second match, and her luck was out as the Wimbledon champion, Krejcikova, battled to a 7-5, 6-4 victory over Gauff, whose place in the semi-finals was already secured.
All three women finished the group stage with two wins from three matches but Krejcikova, the lowest-ranked player at the tournament, topped the pool by virtue of the best sets percentage, with Swiatek the one to miss out.
Krejcikova, of the Czech Republic, will take on the Olympic champion, Zheng Qinwen, in the last four on Friday while Gauff meets the world No 1, Aryna Sabalenka. Krejcikova’s shock Wimbledon win was the outstanding high point of what has been largely a disappointing season for her.
But she has risen to the occasion in Saudi Arabia, bouncing back from a three-set defeat by Swiatek in her opening match by beating Pegula and Gauff, saving 11 of 12 break points on Thursday.
“Definitely this is a big win for me,” said the 28-year-old in her on-court interview. “I’m really happy with the way that I played. Being able to quality for the Finals and also reaching the semi-finals, it’s something unimaginable before this tournament. I’m very proud of myself and I’m looking forward to the new challenges.”