Venus Williams rallied at the WTA Finals to beat Caroline Garcia of France 6-7 (3), 6-2, 6-3 Saturday and become the oldest women's finalist at the year-end tournament.
In the other semifinal, Caroline Wozniacki of Denmark beat Karolina Pliskova of Czech Republic 7-6 (9), 6-3 to set up a championship match against the American.
The 37-year-old Williams said "it's hard to produce your best tennis at the end of the year because you've given so much."
Williams holds a 7-0 winning career record over 27-year-old Wozniacki and has only lost one of 15 sets they've played.
Williams, who appeared in the Australian Open and Wimbledon finals this season, won the WTA Finals in 2008, and was also a finalist in 2009. Wozniacki was a year-end finalist in 2010.
Williams started the semifinal flat but kept improving against Garcia.
"One more match and I'm going to be playing against one of the best players in the tournament, another Caroline, and it's going to be like deja vu," Williams told the crowd. "This tournament is one of the best, the top eight players in the world. Nothing is easy and to be standing in the winner's circle, almost, is so amazing."
Williams became more aggressive and Garcia, making her WTA Finals debut, started to make too many errors. In the end, Williams saved 12 of 13 break points in the match, and took advantage of four of nine break-point opportunities.
The third-ranked Pliskova's loss is good news for Simona Halep, who will retain the No 1 ranking through to next year.