Venus Williams came from a set down to beat Caroline Garcia on Saturday and become the oldest finalist in the WTA's end-of-season competition since Martina Navratilova in 1992.
The 37-year-old American, seeded fifth for the WTA Finals in Singapore, fought her way to a 6-7 (3-7) 6-2 6-3 victory in two hours and 29 minutes.
She won this competition in 2008 and was a beaten finalist a year later up against sister Serena, with Caroline Wozniacki awaiting her in this year's denouement.
The in-form Dane booked her place in the decider earlier in the day by battling past Karolina Pliskova 7-6 (11-9) 6-3.
Garcia took control of the match early on, chasing down a heavy, cross-court Williams rally ball and looping a brilliant forehand over the net post to kiss the line and break her opponent for a 2-0 lead.
Williams responded by breaking straight back with a confident forehand winner down the line and a tight contest stayed on serve after that, although both players were required to dig deep and save numerous break points.
It was the veteran who eventually blinked first, unexpectedly committing successive double faults and missing a forehand to lose the tie-break.
Williams was in trouble at 15-40 down and starring at another break of serve in the first game of the second set, but the seven-time grand slam singles champion displayed her enduring class by saving two more break points before firing off an unreturnable forehand to hold.
Despite seemingly being affected by a hip problem, she then seized on Garcia's sudden uncertainty to break to love and march into a lead that she did not surrender to level the match.
There was nothing between the two players in a cagey third, but it was Williams who earned the crucial break, powering a succession of controlled, heavy forehand returns into the corners to force a Garcia error.
And there was yet another display of resilience in the next game, Williams recovering from 0-40 down to save three more break points; a successful challenge showed a swinging serve down the T had glanced just a fraction of the line for an ace to get it back to deuce.
Garcia had no answer after that, sending a forehand long and then netting a backhand to confirm Williams' progression to the decider.
Williams  bt Garcia  6-7 (3-7) 6-2 6-3
Williams - 33/29
Garcia - 31/30
Williams - 4/10
Garcia - 6/5
BREAK POINTS WON
Williams - 4/9
Garcia - 1/13
FIRST SERVE PERCENTAGE
Williams - 60
Garcia - 65
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Williams - 68/52
Garcia - 63/47
Williams - 109
Garcia - 98