US Open champion Coco Gauff beat Wimbledon winner Marketa Vondrousova to reach the semi-finals of the season-ending WTA Finals in Cancun, Mexico.
The 19-year-old won 5-7 7-6 (7-4) 6-3 against the Czech and will now play fellow American and doubles partner Jessica Pegula in the last four.
Poland's Iga Swiatek beat Ons Jabeur 6-1 6-2 to book her semi-final spot.
She will now play top-ranked Aryna Sabalenka, who defeated Elena Rybakina 6-2 3-6 6-3 to secure her progress.
Gauff's win against Vondrousova made her the first teenager to make the final four of the championships since Caroline Wozniacki in 2009.