Caroline Wozniacki staved off a ferocious fightback from Venus Williams as the Dane claimed the biggest win of her career to take a 6-4, 6-4 victory in the WTA Finals title showdown in Singapore.
The 37-year-old American had held a 7-0 record over Wozniacki going into the match but the Dane put on an inspired display of serving and retrieving to claim the elite eight-woman event for a first time after losing a previous final in 2010.
Wozniacki looked set for a landslide victory after racing into a one set and 5-0 lead. But Williams' warrior spirit came to the fore as she reeled off four straight games to bring herself back into contention. The Dane, 27, held her nerve though, ending her jinx against the five-time grand slam champion having only previously taken one set off her opponent.
"It was all going well at 5-0 in the second set, then she went for her shots and I was just so happy to get it done in the end," Wozniacki, who will rise three places to third in the world rankings after the win, said in a courtside interview.
"Well eight is my lucky number and I figured if I was ever going to beat her it would be today ... and I just went out there and did my best."
"It is really sweet, I couldn't be happier right now."
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Wozniacki has been in brilliant, aggressive form in Singapore, beating Elina Svitolina and Simona Halep at the group stage and Karolina Pliskova in the semi-finals.
After an even start, she seized control of the final by breaking four straight times across the sets to lead 5-0 in the second set.
But with the title in her sights, Wozniacki was unable to serve it out in the seventh game and the ninth, when Williams produced an astounding crosscourt winner to win her fourth straight game.
But Wozniacki finally triumphed in the 10th game with her backhand winner to celebrate in style.
Williams looked increasingly weary as the match wore on a day after a gruelling, three-set semi-final against Caroline Garcia.
In a contrast of styles, counter-puncher Wozniacki tried to extend the rallies while her big-hitting opponent attempted to shorten points through sheer power and rushes to the net.
Wozniacki broke three times in the first set and took a one-set lead when Williams played a loose service game at 4-5 down, just the second set she had ever conceded to the Dane.
Wozniacki, in her first WTA Finals since 2014, flourished in Singapore, with a tough three-set defeat to Garcia her only slip-up.
It completed a stellar comeback season for the two-time Grand Slam finalist, who endured an injury-marred 2016 and finished ranked number 19, her lowest position since 2007.
Williams was denied a 50th career triumph and first for the year, having finished runner-up at the Australian Open and Wimbledon. She will remain the world No 5 after playing in her first WTA Finals since 2009.
Earlier, world No 3 pairing Timea Babos and Andrea Hlavackova won the WTA Finals doubles title on debut with a 4-6, 6-4, 10-5 comeback victory over Kiki Bertens and Johanna Larsson.