Aryna Sabalenka’s preparations for her US Open title defence suffered another blow as she crumbled in a surprise defeat by Sara Bejlek in the fourth round at the Cincinnati Open.
The Belarusian top seed was 5-1 up in a first-set tiebreak and 4-1 in front in the second set, but Czech Bejlek rallied back on both occasions to win 7-6 (9-7) 6-4.
Sabalenka’s position as world number one is now under threat, with Elena Rybakina able to replace her at the top of the rankings if she reaches the Cincinnati final.
The 28-year-old is bidding to become the first woman to win three successive US Open titles since Serena Williams in 2014.
However, her build up to the New York major – which begins on Sunday, 30 August – has been far from ideal with early losses in both Cincinnati and Toronto.
Since winning the “Sunshine Double” at Indian Wells and Miami in March, Sabalenka has reached just one semi-final, which she lost to Jessica Pegula in Berlin.
Meanwhile, it is the first time that Bejlek, ranked 35th in the world, has defeated a top-10 opponent and reached a WTA 1000 quarter-final.
“I totally lost my words. I just keep believing and that was the key today,” said the 20-year-old, who won the Abu Dhabi hard court title in February as a qualifier.
“She played amazing tennis and I was just believing, keep fighting and I was just trying my best and that’s it.”
Bejlek will face former Australian Open champion Madison Keys in the last eight.
In the men’s event on Wednesday, American Tommy Paul saved a match point en route to knocking out top seed Alexander Zverev.
Germany’s Zverev won the first set and trailed 3-4 in the second before the match was suspended for 90 minutes because of rain.
But the French Open champion was unable to convert a match point at 6-5 in the second-set tie-break and Paul forced a decider.
Zverev led 4-2 in the third set before the world number 24 won four straight games to complete a 4-6 7-6 (8-6) 6-4 victory and set up a quarter-final against Italy’s Flavio Cobolli.