World No.1 Iga Swiatek has been dumped out in straight sets at the Cincinnati Masters, after taking aim at the tennis balls the female players have been made to use in North America.
Swiatek was no match for American Madison Keys, who booked her place in the quarter-finals with a 6-3 6-4 victory against the Polish superstar.
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The two-time grand slam champion hasn't got past the round of 16 in four tournaments since winning Roland Garros in June.
It comes after Swiatek hit out at US Open organisers for making women use different balls to the men for the final grand slam of the year, and its many lead-up events.
"We make more mistakes [with these balls], for sure,” Swiatek told the press after her 6-4 7-5 win over Sloane Stephens at the Cincinnati Masters.
“So I don’t think that’s really nice to watch visually I don't know why they are different than men's ones.
"Fifteen years ago probably women had some elbow injuries because the balls were heavier and they changed them to women's balls. But right now we are so physically well prepared that I don’t think it would happen.
“I feel, it’s really hard to control them, but everybody has the same conditions, so we are trying to deal with that. I don’t get why they are different, honestly.
"I think those balls are horrible, especially after like three games of really hard playing, they are getting more and more light.
“At the end, you can’t even serve at 170km/h because it’s going to fly like crazy. I think they are pretty bad.”
In 2019, the situation was even more confusing with the North American swing on the WTA Tour employing the use of different balls at different tournaments.
This year has been a change, with the tournaments across the United States and Canada all using the same ball as the one the women will play with at the US Open.
However, this is still inconsistent with the rest of the year when the same balls are used for men and women in tournaments around the world.
Retired Aussie champion Ash Barty's former coach Craig Tyzzer was also critical of the ball situation at the US Open, claiming his player would never win a slam at Flushing Meadows under those circumstances.
Barty - who won three major titles before announcing her shock retirement this year - never made it past the fourth round at Flushing Meadows.
Swiatek says she and other players have discussed the ball situation with WTA boss Steve Simon.
“I know that many players complain, and many of them are top 10 [players],” Swiatek said.
“Me and Paula Badosa last year talked with Steve about changing to men’s balls. I don’t think it would be a problem because it’s still the same company.
“Maybe we should push a bit more. I stopped pushing and trying to convince WTA because the war in Ukraine happened and I refocused on something else.”
Iga Swiatek fails to move past round of 16 again
Swiatek had been on a 20-match hard court winning streak until eventual runner-up, Beatriz Haddad Maia ended it at last week's Canadian Open.
Haddad Maia's stunning victory also snapeed Swiatek's 23-match winning streak at 1000-level events, following titles at Doha, Indian Wells, Miami and Rome for the World No.1.
Against Keys in Cincinnati, Swiatek dropped the first set and lost the first five games of the second before rallying to win four straight games.
The Pole then fought off one match point before Keys clinched victory with a forehand winner to the ad court off a shot that clipped the net.
Thursday's win was the 24th-ranked Keys' first in six matches against a top-ranked opponent. She had never won a set against a No.1.
Swiatek won her first two matches against Keys, including 6-1 6-0 earlier this year at Indian Wells.
"She beat me pretty badly the last time we played, so I was happy to get the win," said Keys in an on-court interview.
Two-time Wimbledon champion Petra Kvitova also advanced to the quarter-finals with a 6-1 4-6 6-0 win over Wimbledon finalist, fifth seed Ons Jabeur.
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