Defending WTA Finals champion Agnieszka Radwanska comfortably beat Karolina Pliskova 7-5 6-3 in a straight shoot-out for the last semi-final spot.
Both players recovered from defeats in their opening matches by winning their second outing in Red Group, setting the stage for a winner-takes-all clash on Friday.
And it was Radwanska who triumphed, the Pole maintaining her 100 per cent record against Pliskova by coming out on top without dropping a set for the seventh match against the Czech in succession to set up a semi-final clash with world number one Angelique Kerber.
With so much on the line neither player wanted to let anything slip in their early service games, but Pliskova eked out a 4-2 advantage.
Radwanska fought back, defending well and impressing when moving towards the net before claiming the first set with an exceptional forehand return.
The Pole moved a break up at the first attempt in the second set but dropped her next two service games to trail 3-2.
However, Pliskova, paying the price for copious errors, capitulated and failed to convert what proved a pivotal break point in game seven, with Radwanska marching on to a semi-final with top seed Kerber on Saturday.
Radwanska  bt Pliskova  7-5 6-3
Radwanska - 15/8
Pliskova - 27/29
Radwanska - 2
Pliskova - 6
BREAK POINTS WON
Radwanska - 5/8
Pliskova - 3/7
FIRST SERVE PERCENTAGE
Radwanska - 59
Pliskova - 52
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Radwanska - 74/42
Pliskova - 70/36
Radwanska - 63
Pliskova - 53