Caroline Wozniacki survived a "crazy" opening set to reach the WTA Finals title match in Singapore after a hard-fought victory over Karolina Pliskova.
Wozniacki saved three set points in the 10th game of the match and three more after squandering a 6-1 lead in the tie-break, before eventually taking it 11-9 after an hour and 20 minutes.
The two players then traded breaks at the start of the second set before Wozniacki made a decisive move. The Dane broke to love in the eighth game and served out for a 7-6 (11/9) 6-3 win over her Czech opponent.
The outcome ensures Simona Halep will end the year as world number one, with Pliskova now unable to catch her.
" That was a little bit crazy," Wozniacki, 27, said in an on-court interview, shown on BT Sport.
"At 6-1 in the tie-break I feel kind of good about myself out there and then all of a sudden she just started hitting winners everywhere and then I started not feeling so good about myself!
"It was kind of a roller-coaster and I'm just happy that I got that first set under my belt. I think it was really important for both of us.
"I've been playing really great tennis this week and to be in the final here means a lot to me. It feels great."
Wozniacki will face Venus Williams in the final after the American came from a set down to beat France's Caroline Garcia in a match lasting a minute shy of two and a half hours.
Garcia took the opening set on a tie-break but Williams hit back to take the next two and seal a 6-7 (3/7) 6-2 6-3 victory, the 37-year-old recovering from 0-40 down to serve out the match.
Williams is the oldest woman to reach the final of the season-ending event, with the 24-year-old Garcia left to rue the fact that she converted just one of her 13 break points.
"She competed so well this whole tournament and every time I thought I was getting close she would play amazing," Williams said in her on-court interview.
"It's not over till the fat lady sings and I'm not fat. I watch my figure!"