"It's really unbelievable for me", said Siniakova, who was runner-up at the Swedish venue in 2016.
Caroline Wozniacki struggled with injury on Sunday as she lost the Swedish Open final to Katerina Siniakova. "It's one of the tournaments where I've done my best, and it's one of my favorite tournaments of the year".
Although Wozniacki briefly got the match back on serve, Siniakova immediately broke again and served out the opening set. "I'm definitely going to try and make this one". She'd been seeking her 26th career title but first on clay since 2011, when she was the world No. 1. The 31-year-old Italian, ranked 31 in the world, was playing in his 13th ATP final. "This week was really tough for me", said Fognini.
Wozniacki has enjoyed a recent resurgence and the Dane will be optimistic about her chances at the US Open next month, where she made the semi-finals past year.
The top seed, who has won more matches than anyone else in 2017 without yet taking a title, took all but four games against Ukrainian qualifier Kateryna Kozlova. Harrison beat fifth-seeded Briton Kyle Edmund 6-7 (5/7), 6-3, 6-4 to advance.
She was broken in the first game of the match by the 21-year-old World No.55 but rebounded and eventually claimed the set on her fourth set point when she drew a forced forehand error from Mertens. Fellow Chinese and top seed Zhang Shuai and European stars Jelena Jankovic and Kristyna Pliskova all tumbled out early to leave Peng as the sole remaining seed in the semi-finals.