Elina Svitolina is through to the final of the WTA Finals after beating Kiki Bertens 7-5 6-7 (5-7) 6-4.
Svitolina, 24, overcame the eighth seed in two hours and 38 minutes, the longest match of the tournament.
The 24-year-old is the first Ukrainian to reach the WTA Finals final and will face either Sloane Stephens or Karolina Pliskova, who contest the other semi-final on Saturday.
The final takes place at 12:30 (BST) on Sunday.
The pair traded early breaks in the first set but it then followed serve until Svitolina broke in the final game to go ahead in the match.