Petra Kvitova sent defending champion Angelique Kerber packing from the rain-affected Sydney International on Thursday to book a semi-final spot and get an edge over her rival heading into the Australian Open next week.
It was always going to be a tight match with the pair tied 6-6 in 12 previous meetings.
Kvitova kept the pressure on and got a crucial break in game four of the second set and there was no way back for Kerber who meekly surrendered 6-4, 6-1.
Her reward is a clash with Belarusian qualifier Aliaksandra Sasnovich, who beat Switzerland's Timea Bacsinszky — on the comeback trail after a 14-month winless spell — 6-3, 6-3.
Australia's Ashleigh Barty followed up her giant-killing wins over world number one Simona Halep and former French Open champion Jelena Ostapenko by easily accounting for 10th seeded Belgian Elise Mertens 6-3, 6-3.
The 22-year-old is now on a six-match win streak, dating back to her career's biggest title at the WTA Elite Trophy in Zhuhai.
She will play seventh seed Kiki Bertens who blasted past Yulia Putintseva 6-2, 6-2 for a place in the final.
In the men's draw, veteran Andreas Seppi derailed top seed Stefanos Tsitsipas' title hopes.
Seppi, 14 years older than his 20-year-old Greek opponent battled back for a gutsy 4-6, 6-4, 6-4 win.
Seppi will now play third seeded Argentine Diego Schwartzman, who needed to fight hard to beat Yoshihito Nishioka 6-1, 3-6, 7-6 (7/1)
In other quarter-final action, fifth seeded teenager Alex De Minaur continued his strong start to the season with a 7-6 (7/4), 6-3 victory over fellow Australian Jordan Thompson.