Sabalenka, who had cruised through the previous round, was ambushed by the 20-year-old, whose movement left the top seed searching for answers in a Wednesday night contest completed shortly after the clock ticked past midnight into Thursday morning.
Sabalenka was up a break in the 65-minute opening set and then led 5-1 in the tiebreaker before Bejlek roared back.
Bejlek broke the Belarusian star three times in the second set and secured the win with a love service game capped by an ace.
“I lost my words — it feels incredible,” Bejlek said after making her first win over a top-10 player a triumph over the No. 1.
Bejlek, who won the Abu Dhabi hard court title in February as a qualifier, booked a quarter-final meeting with American Madison Keys.
It was yet another setback for Sabalenka as the start of the US Open looms on August 30.
The four-time Grand Slam champion was ousted in the fourth round of the Toronto Masters last week, having lost in the same round at Wimbledon and in the quarter-finals at Roland Garros. (AFP)