Karolina Pliskova booked her place in the WTA Finals last four with a 6-3 6-4 victory over Petra Kvitova, ending the latter's hope of qualification.
Kvitova needed a straight-sets win to stand any chance of making it through to the semi-finals but the chance of that quickly evaporated as errors blighted her game.
There was more fight in the second set but she let a 4-2 advantage slip as Pliskova reeled off four successive games to guarantee her place in the next round.
Who joins her from the White Group will be decided in Thursday's second match between Elina Svitolina and Caroline Wozniacki – the Dane needing to win in two to oust her Ukrainian opponent.
An out-of-sorts Kvitova struggled from the outset with three double faults in her first service game, and wayward groundstrokes in game four allowed Pliskova to open up a commanding 4-0 lead.
The errors quickly switched to Pliskova in game five but Kvitova needed four break points before finally getting on the board – a stinging forehand clipping the line to bring some relief to the two-time Wimbledon champion.
Fourth seed Kvitova showed great resolve to save three openings for Pliskova in the sixth and battled through to make her fellow Czech serve for the set.
Another unforced error from the left-hander gave her that advantage, a long forehand putting Pliskova 1-0 ahead and ending the world number five's faint chances of qualification.
With her chances over, Kvitova played with more freedom at the start of the second set and raced into a 3-0 lead, a beautiful drop shot followed by a Pliskova miss giving her an early break.
Winners were fizzing off Kvitova's racket and at 4-2 she looked on course to level the match and leave her opponent's progression hanging in the balance.
However, the world number eight came storming back with an array of quality groundstrokes, mixing up her game with aplomb to frustrate her Fed Cup team-mate.
A third hold of the set was enough to put Pliskova into Saturday's semi-finals, converting her second match point to record a first win over Kvitova in four attempts.
Pliskova  bt Kvitova  6-3 6-4
Pliskova – 6/17
Kvitova – 22/32
Pliskova – 1/1
Kvitova – 4/7
BREAK POINTS WON
Pliskova – 5/9
Kvitova – 3/8
FIRST SERVE PERCENTAGE
Pliskova – 68
Kvitova – 58
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Pliskova – 56/57
Kvitova – 52/32
Pliskova – 67
Kvitova – 52