NEW YORK—Jimmy Connors played in 22 U.S. Opens, a men’s Open Era record, and reached a record 12 straight semifinals from 1974 to 1985. But one participant has shown a longer shelf life than the brash baseliner in bouncing around the Flushing Meadows grounds year after year.

Wilson balls have been the only balls used in competition since the tournament moved to Flushing Meadows from Forest Hills in 1978. Wilson ships about 90,000 balls to the USTA Billie Jean King National Tennis Center each August for use in practice, qualifying rounds and main-draw matches. The USTA estimates it uses about 70,000 balls throughout the event. When the tournament ends, the balls behave a little bit like players with busy schedules, scattering from the site to hit the next court. The USTA donates the balls for reuse to community and youth organizations throughout the country.

While their rubbery resiliency, optic-yellow fuzzy exteriors and trademark bright red U.S. Open tattoos make them look like bouncy identical twins, there is some difference between the extra-duty felt Wilson ball used by the men and the regular-duty felt Wilson ball used by the women. And contrary to popular perception, there is no weight disparity in the two balls, according to Wilson.

"The Wilson U.S. Open extra duty and regular duty balls weigh the same. But it’s the regular duty ball’s performance through the air that creates the sensation that they’re lighter," says Jason Collins, Wilson Global Business Director of Tennis Balls. "Sometime people think ‘the balls are extra duty then they must be heavy duty,’ but that’s a misconception. The real difference is dynamic performance—the static measurements are all the same. If we dropped the two balls from the same height, they would bounce to the same height, they would weigh the same. The difference is regular duty balls play tighter because of minimal fluffing, this results in quicker performance through the air. The extra duty ball has additional fluff, which results in the ball sitting on the strings longer, creating a sensation that the ball is heavier."

Players grow accustomed to adjusting to different balls at different times throughout the season—Dunlop is the official ball of Masters Series clay-court events before Babolat takes over as the official ball of Roland Garros—and the quest for continuity is one reason why women play with a regular-duty ball at the U.S. Open.

"Part of the WTA by-laws is that the WTA players use regular-duty balls in their events, so the U.S. Open women’s matches use the regular-duty ball, which is consistent with what women use throughout the year," Collins says.

The next time you watch a U.S. Open match, look for the shoulder tube that chair umpires carry onto court. It typically contains about 10 cans of balls, and an umpire will usually open five of them (four for play and one for replacement balls) at the start of the match. At the Open, fans are allowed to keep stray balls that scamper into the stands, though you don’t have to be at Flushing Meadows to gain the U.S. Open ball experience.

"The Wilson U.S. Open balls you buy in the store are really identical to the ones pros use at the U.S. Open," Collins says. "When you look at ITF and USTA specifications for tennis balls, there are four areas that are measured — rebound, size, weight and deformation. While U.S. Open balls for every day play need to meet each of these criteria, tournament production has very little variance from ball to ball. Using rebound as an example, all balls need to bounce between 53-58 inches when dropped from a certain height. While in regular production the variance is still small, we inspect 100 percent of tournament balls to ensure there is as little variance as possible. This guarantees that players will experience the same ball performance over the two week event — the first ball hit plays identical to the last ball hit. So when Roger Federer goes out on court at the Open, any ball he plays is going to play virtually identical. The U.S. Open ball is used in USTA and rec leagues all year, so if you’re playing with the U.S. Open ball at your club, you’re playing with the U.S. Open tournament ball."

Which ball should you use? That depends on the surface you play on, if you’re playing indoors or outdoors and, of course, personal preference.

"A Wilson regular-duty ball is largely designed for clay-court play and indoor play. It’s for clay-court play because it’s a tighter weave of the felt, so it doesn’t pick up as many of the clay particles and won’t fluff up as much," Collins says. "There’s a lot of static electricity in indoor court buildings, so again, the regular-duty ball won’t fluff up as much. Typically, extra duty is sold more in the U.S. because outdoor public park hard courts are where a lot of frequent players play."