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.
“It feels amazing, I can’t believe I am here as the winner,” she told BT Sport.
“Venus Williams is a great champion and she made it very, very difficult for me. This is my biggest title to date.”
Simona Halep will end 2017 as world number one, becoming the 13th women to earn the honour.
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.
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.
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.