Aryna Sabalenka's US Open title defence preparations suffered another setback after a stunning fourth-round defeat by Sara Bejlek at the Cincinnati Open. The Belarusian top seed, who was 5-1 up in a first-set tiebreak and 4-1 ahead in the second set, collapsed to lose 7-6 (9-7) 6-4 to the Czech world number 35.

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.

The defeat marks another early exit for Sabalenka, who has struggled since winning the 'Sunshine Double' at Indian Wells and Miami in March. Meanwhile, it is the first time 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.

In the men's event, American Tommy Paul saved a match point to knock out top seed Alexander Zverev, setting up a quarter-final against Italy's Flavio Cobolli.