World No 1 Aryna Sabalenka was sent spinning out of the fourth round of the ATP-WTA Cincinnati Masters, beaten 7-6 (9/7), 6-4 by 35th-ranked Czech Sara Bejlek.
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 quarterfinal meeting with American Madison Keys.
It was yet another setback for Sabalenka as the start of the US Open looms on 30 August.
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 quarterfinals at Roland Garros.