Naomi Osaka beat Petra Kvitova 7-6 (7-1) 4-6 6-4 in a gruelling opening match of the WTA Finals in Shenzhen, which lasted two hours 39 minutes.
Japan's world number three Osaka won the first-set tie-break after Kvitova missed three break points at 4-4.
But the Czech, 29, fought back from a break down to take the second set and force a decider.
Kvitova made Osaka wait for her win, breaking the 22-year-old when she served for the match at 5-2.
"For me it was very difficult and usually there's first-round first jitters and you can't really afford it as she came hard at me from the beginning," Osaka said.