Victory for Boulter comes as she battles to keep the British number one ranking with Emma Raducanu and Sonay Kartal closing in.
Boulter has been the nation’s leading women’s player since June 2023, but has dropped down the world rankings in a stop-start season.
Raducanu is up to 49th after reaching the Miami quarter-finals last month, while Kartal’s rapid rise has moved her up to 60th.
Both play their Madrid first-round matches on Wednesday.
“I don’t feel any pressure at all,” Boulter told BBC Sport before beating Siniakova.
“I’m very comfortable in the seat that I am and I know that every single person has a different journey and my journey is not going to be compared to anyone else’s – I’m proud of that.
“I’ve been the British number one for two years, it’s been a fair while now. It’s something which I’m quite used to.”
After winning one and losing one of her Billie Jean King Cup singles matches last week on indoor clay in the Netherlands, Boulter went to Alicante – where her fiance Alex de Minaur has long been based – for a training week.
She decided that was the sensible option – instead of squeezing in another WTA clay-court event before Madrid – as she continues to manage a foot injury.
The issue has left her only able to play five tournaments this season, missing almost two months of the WTA Tour between the Australian Open in January and Indian Wells in March.
“I want to see the girls going higher and higher [in the rankings], but personally I have my own goals and try to focus on those a lot,” added Boulter, who secured GB’s place in the Billie Jean King Cup Finals alongside Jodie Burrage in the deciding doubles.
“For me I just need to try and stay healthy. That’s that most important thing and then the ranking will take care of itself.”
