Plíšková, currently eighth in the Race to Singapore standings, is battling for one of the three available spots in the season-ending tournament starting on October 21.

World number 101 Boulter, facing a top-10 player for the first time, took her Czech opponent by surprise with two breaks of serve to clinch the opening set.

Plíšková is known for her powerful serve, but it was her return game which came to the rescue as she converted nine break points to move past Boulter into the semifinals.

“(In the second set,) I started with a quick break, which always helps a little bit, and I improved my serve,” world number six Plíšková said.

“I played more aggressive, I thought, and she didn’t have time much to play her game, and that was the difference.”

Earlier in the day, fourth seed Aryna Sabalenka suffered a shock 7-6(2) 7-6(5) defeat by Swiss Timea Bacsinszky, which ended the Belarusian’s chances of participating in the WTA Finals.

Having recently returned to action from a serious leg injury, Bacsinszky deployed the full range of her groundstrokes to reach her first WTA semifinal since the French Open last year. The 29-year-old will face Plíšková for a place in the final.

France’s Caroline Garcia also booked a spot in the last four after her sixth-seeded opponent Petra Martić retired while trailing 6-2 1-0.