Aryna Sabalenka, the world number one, suffered a 7-6 (9/7), 6-4 defeat to 35th-ranked Czech player Sara Bejlek in the fourth round of the ATP-WTA Cincinnati Masters.
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 Feb. as a qualifier, booked a quarter-final meeting with American Madison Keys.
It was yet another setback for Sabalenka as the start of the U.S. Open looms on Aug. 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.
The women's 2023 champion Coco Gauff saved all eight break points she faced in a 6-3, 6-2 win over Czech Marie Bouzkova. The fourth seed double-faulted on her first match point but leapt up to smash an overhead winner on her second chance.
"I played well the whole match, going for my shots, coming forward and not letting her counter-punch," Gauff said. "I'm happy with how I played."
Defending women's champion Iga Swiatek needed four match points to get past Diane Parry of France 6-3, 6-3. The seventh-seeded Pole now faces a re-run of last week's Toronto final against Elena Rybakina, who beat Diana Shnaider 6-4, 6-4.
Six-time Grand Slam champion Swiatek captured her first title of 2026 with her victory over reigning Australian Open champion Rybakina in Toronto.
Two other current Grand Slam champions bowed out. Ukraine's Marta Kostyuk rallied from a set down to beat French Open winner Mirra Andreeva 4-6, 6-0, 6-2. Wimbledon champion Linda Noskova fell to Amanda Anisimova 6-1, 6-4 in another weather-disrupted match.