Wimbledon champion Linda Noskova dismantled Coco Gauff to become the first player to reach the last four of the WTA Finals.
Czech Noskova played with controlled aggression and served brilliantly to beat the American 6-3 6-3.
World number eight Noskova was composed throughout and broke Gauff’s serve four times in the first set alone.
She sent powerful winners to all corners to outmuscle her opponent, who is one of the best movers on the WTA Tour, and committed only one unforced error in a dominant second set.
The 21-year-old fittingly sealed victory with an ace to secure a place in the semi-finals on her tournament debut.
“Thank you guys for the cheers I’m getting now – but it was a little weaker in the match, guys, I’m not going to lie,” Noskova joked to the California crowd afterwards.
“I feel like I stayed in the match through every moment, from beginning to end.”
It is a sixth top-10 victory of the season for Noskova, who won her maiden Grand Slam at Wimbledon in July.
Gauff, who won a thriller against compatriot Jessica Pegula on Sunday, had a poor day on serve.
She hit the same amount of aces and double faults – eight apiece – and won only six of 24 points behind her second serve.
Fourth-ranked Gauff can still qualify for the last four, as can Pegula.
Earlier in group one, Pegula claimed her first win since September to eliminate alternate Marta Kostyuk.
American Pegula won 6-2 7-5 in a scrappy match that saw seven breaks of serve.
Ukraine’s Kostyuk, who has been dealing with a wrist injury since August, only found out she would be playing on Saturday after world number one Elena Rybakina withdrew.
She lost her opener to Noskova on Sunday and struggled for rhythm and consistency in a low-quality match against Pegula.
Pegula made only five unforced errors to Kostyuk’s 17 as she took the first set, and the home favourite quickly went up a break in the second.
However, four breaks of serve then ensued, with Pegula eventually sealing the decisive break at 5-5 before serving out victory.
Elsewhere, Britain’s Toby Samuel fell 6-1 6-4 to world number 25 Lorenzo Musetti in the Basel Open second round.