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The top seed won through 7-5, 4-6, 6-2 against Belgian Elise Mertens and next plays Siniaková who advanced past French third seed Caroline Garcia 6-2, 7-5.
"It was a difficult match," said Wozniacki, bidding for her 26th career title and first this season against 21-year-old Siniakova.
"She played very well, aggressively in the wind and the sun was very low," said the Dane. "It's a positive that I put myself in the sitation where I'm in a fifth final. I'm definitely going to try and make this one."
The win gives Wozniacki a first final on outdoor clay since 2011 when she was world number one.
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Siniakova, ranked 56th, ousted French Open quarter-finalist Garcia in a match interrupted three times because of the rain.
The Czech player made her first WTA final in Bastad a year ago, and has since won her first title this year in Shehzhen.
"It's amazing. The atmosphere is really good year, so I'm happy I could come back. I wasn't sure if I would play here again, but I'm back and in the final!"