Last year was one of steady progress for Siniakova, and she signaled her intent to continue this rise with a 6-3, 4-6, 7-5 victory over the World No.4.
"I'm really happy that I won for me, because it was a very tough match," Siniakova said. "When I walked onto court I was feeling that I could win and I just wanted to show my best.
"I was feeling a little bit of pressure in the final set and I'm so happy that I manged to come through and win the match."
Siniakova's only previous Top 20 victory came against Andrea Petkovic in 2015, but she held her nerve during a tense finale, adding a more significant victim to her ledger when Halep's forehand found the net.
Now Siniakova has the chance to serve for the match! pic.twitter.com/9wVn6LakQW
? WTA (@WTA) January 4, 2017
Siniakova will now face fellow 20-year-old Nina Stojanovic for a place in the semifinals. "Of course I feel good on the court; I worked a lot in the off-season and I can say that my confidence is growing and I hope it will continue like this in the next round," Siniakova added.
Halep, who is not scheduled to play another Australian Open tune-up, will head to Melbourne a little earlier than anticipated. "I think she played really well, she's a strong player. I'd never played against her before, she hits the ball hard, served good, so she deserved to win today.
"I was struggling a little bit. I didn't really feel my game, but there is no excuses for that. She managed to be stronger on court and that's why she could win. It was hot, but I'm used to this heat already - I've been to Australia practicing. It's a little bit tiring because it's the second match of the year after a two-month break."