Dutch ace Kiki Bertens will take on Donna Vekic for the St Petersburg Ladies Trophy after each recorded straight sets semi-final victories.
Second seed Bertens defeated fourth-seeded Belarusian Aryna Sabalenka, the 20-year-old who at 10th is two places below her in the world rankings, 7-6 (7-5) 6-2 in an hour and 38 minutes on Friday.
"It was a really tough match. She's a great player for sure," the Dutchwoman said in her on-court post-match interview.
"She is hitting the ball really hard so I really had to stay aggressive where I could and where I could just try to get the ball back."
Croatia's eighth seed Vekic, conqueror of top seed and defending champion Petra Kvitova in the last eight, joined Bertens in Sunday's final after she got past veteran Russian Vera Zvonareva 6-2 6-2.
Vekic has a 3-1 lead over Bertens in their head-to-head record, which includes victory for the former in the second round of the Brisbane International last month.
The final of the Thailand Open in Hua Hin will be contested between Australia's Ajla Tomljanovic and Ukrainian teenager Dayana Yastremska.
Sixth seed Tomljanovic triumphed 6-4 6-1 against Slovenia's Tamara Zidansek to advance into her fourth WTA singles final - she is yet to win one.
The 18-year-old eighth seed Yastremska won her semi-final against Poland's Magda Linette 6-4 6-3.
Yastremska, who is looking to add to her maiden WTA title won in Hong Kong last October, had made it through to last four in Thailand by defeating top seed Garbine Muguruza on Friday.
