Karolína Plíšková of the Czech Republic and Elina Svitolina of Ukraine won their opening games of the Women's Tennis Association (WTA) Finals in Kallang in Singapore.
The season-ending event is a round-robin tournament consisting of the eight top-ranked players on the WTA tour.
Today's action in the white group saw world number five Plíšková play world number two and defending champion Caroline Wozniacki of Denmark, with the underdog triumphing 6-2, 6-4 at the Kallang Tennis Centre.
"It's always tough matches against Caroline," the Czech said.
"The last time we played was here, and I remember the first set was more than an hour, and I had set points.
"I believed I had a chance because we had a very good match here last year and I tried my best today."
Svitolina met Petra Kvitová of the Czech Republic, winning 6-3, 6-3.
"I was playing good today, quite solid from the baseline and putting pressure on Petra today," Svitolina said.
"This was the most important thing and I think the key."
Tomorrow's matches are both in the red group.
Japan's Naomi Osaka squares off against Sloane Stephens of the United States and Germany's Angelique Kerber plays Kiki Bertens of The Netherlands.