WTA Finals: Caroline Wozniacki beats Venus Williams to win season finale
Denmark's Caroline Wozniacki beat Venus Williams to claim the biggest title of her career at the WTA Finals in Singapore.
Williams - at 37, the oldest woman to ever reach the final - lost 6-4 6-4.
After a tight first set, number six seed Wozniacki stormed into a 5-0 lead in the second.
Williams then won four straight games but Wozniacki, 27, converted her second Championship point to beat the American for the first time.
"I couldn't be happier right now," said Wozniacki, who slipped to 74th in the rankings last year as she struggled with injury problems.
"It's been an amazing year. Last year was tough with injuries but I fought my way back and to stand here and win the title means a lot to me."
Simona Halep will end 2017 as world number one, becoming the 25th woman to earn the honour since the computer rankings were introduced in 1975.
The Romanian first moved top of the rankings during the China Open at the beginning of October and was assured of her position when closest challenger Karolina Pliskova lost to Wozniacki in Saturday's semi-finals.
Wozniacki finally beats Williams
Fifth seed Williams went into Sunday's final with a 7-0 record against Wozniacki stretching back to 2007, and having won just one set against the American.
The world number five has been in fine form this year and was a finalist at the Australian Open and Wimbledon.
But the veteran will finish 2017 without a title after Wozniacki prevented her from regaining the Finals trophy she won nine years ago.
"I didn't seem to come up with my best tennis until it was too late," said Williams.
"I love being here, it is an honour. Only eight players get to be here and I hope to return."
Former world number one Wozniacki saw huge success on her serve on the slow surface in Singapore, ending with a tournament-leading 21 aces.
And the Dane backed that fine serving up with only eight unforced errors in the final.
It appeared to be one-way traffic in the second set, with Williams looking heavy in the legs and no longer able to attack Wozniacki at the net.
But the 37-year-old rallied to produce some of her best tennis and turn the tide.
With Wozniacki serving for the title at 5-3, Williams outlasted her younger opponent in a gruelling rally before then hitting a sublime forehand passing shot to get back on serve.
But it was Wozniacki's day though and, after hitting her first Championship point long, a perfect backhand saw her break serve and seal her second title of 2017 following last month's Pan-Pacific Open in Tokyo.
It completes a fine 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.
"I am so happy I managed to win in the end," said Wozniacki.
"Eight is my lucky number. If I was going to beat her once in my career, it had to be today."