WTF Fun Fact – Heavy-Duty or Regular-Duty Tennis Balls

At the US Open and other US-based tournaments, the men and women play with different tennis balls. The men play with the slower heavy-duty felt while the women play with the faster regular-duty felt. – WTF Fun Facts

Source: https://www.wsj.com/articles/at-u-s-open-do-women-play-with-faster-tennis-balls-11565957964