Karolina Pliskova beat defending champion Caroline Wozniacki 6-2 6-4 on the opening day of the WTA Finals in Singapore.
Czech world number eight Pliskova, 26, broke early in both sets to win in one hour 32 minutes in the White Group.
Australian Open champion Wozniacki, from Denmark, failed to win any of the 10 break points she created.
Ukraine's Elina Svitolina beat Czech Petra Kvitova 6-3 6-3 in the same group earlier on Sunday.
The event features the leading eight players this year, split into two groups of four.
The top two players from each round-robin group will progress to the semi-finals, with the final on 28 October.
In the Red Group on Monday, Japan's US Open champion Naomi Osaka plays American Sloane Stephens, while top seed Angelique Kerber, from Germany, meets the Netherlands' Kiki Bertens.
The Finals are being held in Singapore for the fifth and final time before moving to Shenzhen in China.