Aryna Sabalenka roared past Coco Gauff to avenge her French Open final defeat and secure her place in the WTA Finals last four alongside Jessica Pegula.
Sabalenka, Gauff and Pegula went into Thursday's matches in Riyadh knowing only two of them would progress from the Stefanie Graf Group.
American Pegula won 6-2 6-3 against already-eliminated Jasmine Paolini, meaning a straight-set win for defending champion Gauff would have knocked world number one Sabalenka out.
Gauff also had the statistical edge, having come out on top in their previous meeting at Roland Garros in June - with Sabalenka putting in a performance she later described as the "worst final I ever played".
Sabalenka was wayward at the start but rallied to beat the American 7-6 (7-5) 6-2 and top the group with a 100% record.
She will play Amanda Anisimova on Friday in a repeat of September's US Open final, which Belarusian Sabalenka won, while Pegula takes on Elena Rybakina.
"We always have great battles. It's enjoyable playing against her because I know it's going to be a great fight," Sabalenka, 27, told Sky Sports.
"Whenever I focus on myself and what I have to do, without getting over-emotional and just staying in the zone - that's the key for me."
Gauff's exit means there is guaranteed to be a first-time WTA Finals champion on Saturday.
French Open champion Gauff did not do too much wrong in the opening set but Sabalenka excels in tie-breaks, having won 22 of the 24 breakers she has played this year.
Gauff's serve, which has been problematic in recent times, held up extremely well in the first set - with 72% of first serves in, 62% of first-serve points won and 70% of points won behind her second serve.
However, she failed to serve out the opener at 5-4 up before her first double fault of the match gifted four-time major singles champion Sabalenka two set points in the tie-break.
On the other side of the net, Sabalenka demonstrated the full range of emotions as she twice battled back from a break down; frustration after several points lost, despair after sending a backhand volley wide during the tie-break and then celebration as she thrashed a powerful forehand winner to clinch it.
Sabalenka quickly moved 4-0 up in the second and, despite briefly giving back one of her two breaks of serve, she served out victory to stay on course for a first WTA Finals title.
Sabalenka is into the semi-finals for a fourth successive year but has only reached the final once, losing to Caroline Garcia in 2022.
An unwell Paolini, who has not had a rest day in Riyadh because of her participation in the doubles tournament with Sara Errani, battled hard against Pegula but the 2024 US Open finalist was ultimately too strong.
Pegula broke twice in both sets as she wrapped up victory in just 63 minutes.
Paolini could not make any inroads in her returning games, with 2023 WTA Finals runner-up Pegula winning 33 of 41 points on her own serve and facing only one break point.
"I served really well, played solid and was aggressive when I needed to be," the 31-year-old said. "There weren't many negative notes."
In the doubles, Timea Babos and Luisa Stefani secured their semi-final spot with a 2-6 7-5 10-5 victory against defending champions Gabriela Dabrowski and Erin Routliffe.
Katerina Siniakova and Taylor Townsend were already guaranteed to progress from the Liezel Huber Group, having won their first two matches.

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