Defending WTA Finals champion Caroline Wozniacki last defeated Petra Kvitova four years ago, the first time the season finale was held in Singapore.
After losing four straight matches since then to the Czech - one each year from 2015 - Wozniacki finally got her win yesterday.
The Australian Open champion beat Kvitova 7-5, 3-6, 6-2 at the Singapore Indoor Stadium for her first win of this year's BNP Paribas WTA Finals Singapore presented by SC Global.
The win keeps her hopes of progressing to the semi-finals alive.
"I know Petra's patterns, but it doesn't always help... she plays so fast that sometimes you just don't have time," said the world No. 3, who trailed their head-to-head count 5-8 before yesterday's match.
"But I think it worked well for me today and I just tried to make her feel uncomfortable."
And she did.
Caroline Wozniacki made 26 fewer unforced errors than Petra Kvitova.
TODAY'S ORDER OF PLAY
Red group: Angelique Kerber (Ger) v Naomi Osaka (Jpn)
Red group: Sloane Stephens (USA) v Kiki Bertens (Ned)
Tickets on sale at wtafinals.com
Last night, the fifth-ranked Kvitova pumped her fist, slapped her thigh and threw her racket but still could not wear her Danish opponent down throughout the 2hr 19min match.
Though Wozniacki was moved all over the court, her defence - and energy - never waned. She played more aggressively when needed, and a Kvitova service return that sailed long sealed the win.
"Against Petra, her pace is so powerful, you just have to stay on your game and you never know what's going to happen, so I just tried to stay focused and get all the balls back," said the Dane, who was still catching her breath during the on-court interview.
Last night's victory was reminiscent of her defeat of Venus Williams en route to winning last year's WTA Finals. Then, she had beaten the American for the first time after eight attempts.
Said Wozniacki: "I think I just always believe in myself when I go out on court. I believe I can win the match. I believe I'm going to win and then if it's not going your way, you try and find a Plan B. And if anyone beats you, it's just because she was better than you that day and you respect that.
"So I was just out there, had the belief and fought till the end."
Kvitova, who hit 40 unforced errors to Wozniacki's 14, felt her opponent had started the third set better.
"We had lots of long rallies and unfortunately I couldn't make those points on my side," said the Czech, who lost her opening match to Elina Svitolina on Sunday.
She will face Karolina Pliskova in the last round-robin match of the White Group, while Wozniacki takes on Svitolina, who beat Pliskova 6-3, 2-6, 6-3 yesterday.
The Dane has a losing record of 1-3 against the Ukrainian, though after last night, this should count for little.
SINGLES ROUND ROBIN
Singapore Indoor Stadium, StarHub Ch203, 7.30pm