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“I think those balls are horrible,” Swiatek said last week.

“They are pretty bad. I know there are many players who complain, and many of them are top 10.”

The US Open is the only major where men and women use different balls and, in the build-up to this year’s tournament, a number of players have echoed Swiatek’s comments.

There has also been a mixed response to the decision to allow in-play coaching at the tournament for the first time.

What difference do the balls make?
The women use Wilson US Open regular duty balls in New York, while the men use Wilson US Open extra duty balls.

The regular duty balls are thinner and lighter than the extra duty ones. Wilson states that the regular balls are likely to fluff up more, external, meaning they play faster but are “less durable” than the extra duty.

Wilson also says that the regular duty balls are developed for softer surfaces, like clay or indoor courts, compared to the extra duty ones, which are for hard courts and “abrasive” surfaces.

Swiatek explained that the lighter balls can contribute to more unforced errors which, in turn, can make matches more unappealing to watch.

“Especially after three games of really hard playing, they are getting more and more light,” she said at the Cincinnati Open last week.

“At the end, you can’t even serve at 170kmh [105mph] because you know it’s going to fly like crazy. They are pretty bad.

“Right now we play powerful, and we kind of can’t loosen up our hands with these balls. We make more mistakes, for sure, so I don’t think that’s really nice to watch visually.”

The US Tennis Association said “a number of factors” are considered in deciding which balls to use.

“The USTA works closely with the WTA and ATP Tour, their player councils and our brand partner on an annual basis to determine what type of balls they recommend,” a USTA spokesperson said.

“The USTA will continue to follow the recommendations of the tours and their player councils to determine which balls are utilised during the US Open.”