Karolina Pliskova won the first set to eliminate Petra Kvitova and then won the second set to advance to the semifinals at the WTA Finals.
Pliskova, who advanced from her group for the second straight year, beat Kvitova 6-3, 6-4 on Thursday to improve her record to 2-1 at the season-ending tournament.
It was Pliskova's first victory over Kvitova in their fourth meeting.
"I knew I lost my other matches to her. I knew this time I had a chance because I was playing pretty well in my other two matches here," Pliskova said on court after the match. "Everything is working well for me this week so I'm just happy."
Kvitova jumped out to a 3-0 lead in the second set, but Pliskova then won six of the next seven games. Kvitova finished the group with three straight losses.
Kvitova, who is from the Czech Republic, rarely loses to players from her country. Since the 2012 Madrid tournament, the two-time Wimbledon champion has won 27 of 29 matches against Czech opponents.
Her only other loss to a Czech player in that time period was to Pliskova's twin sister, Kristyna, at this year's tournament in Charleston, South Carolina.
In the other group match, Elina Svitolina (2-0) will face defending champion Caroline Wozniacki (1-1). Svitolina needs only to win one set to advance.
