Why high-altitude tennis balls will play a big role in this year's WTA Finals

GUADALAJARA, Mexico – Before she arrived here for this year’s WTA Finals, Iga Swiatek spent a few days with Bethanie Mattek-Sands in the mountains near Phoenix, Arizona.

Swiatek mused that she ate well and “didn’t party too hard” and, above all else, practiced with Mattek-Sands in the high altitudinal conditions in preparation for the elevation in Guadalajara, which sits nearly 5,200 feet above sea level.

On Tuesday, Swiatek, stationed on a balcony adjacent to the practice courts for her media day session, was asked how she would assimilate to the high grounds and whether she had any concerns.

“I would say if we would play here with normal balls, they would fly like crazy, so I'm pretty happy that the tournament provided us with the different ones so we can actually play,” Swiatek said.

This year’s balls, provided by Wilson Tennis, are appropriately called high-altitude balls. They are customized to adapt to an environment low on oxygen and high in rebound. While the physical properties of the ball, including the weight, remain largely unchanged from a regular ball, it might feel more dense because of what Wilson describes as deformation (i.e., the ball’s hardness).

In other words, the core of the ball, which is made of natural rubbers, is adjusted to balance deformation with the environment. Think of it this way:

“If you were to test a regular-altitude ball in, say, Chicago or somewhere at sea level, and then test a high-altitude ball in Guadalajara or Albuquerque, New Mexico, the playing characteristics would feel the same,” said Chloe Lee, the chief principal engineer for Wilson tennis balls.

Without those adjustments at a high elevation, the ball would, as Swiatek said, “fly like crazy.”

More specifically, a regular-altitude ball at around 4,000 feet or higher would bounce in the neighborhood of three inches above normal rebound. It would feel like playing with a tennis ball that is twice as pressurized as it is at sea level, according to Lee. On the other hand, a high-altitude ball in Chicago under normal conditions would bounce too low.

“Technically, the cause in rebound change comes from the pressure,” Lee said. “So we toggle the core formula just a little in order to make sure the deformation is the same at both of these levels. Because as you decrease the internal pressure, the ball is going to play softer.”

Martina Navratilova, an eight-time champion at the WTA Finals, understands the pressure these players are under in an event that is one of the hallmarks of the season. Adapting to largely unfamiliar conditions in a high-profile competition will be critical to success.  

“It’s the biggest unknown in this tournament,” Navratilova said, referencing the host city’s elevation. “Most of these players have little or no experience playing at this altitude. I know the ball flies, and it can be really tricky.”

Altitude is not the only component in deciding which type of tennis ball is better suited for competition. Temperature plays a large role as well.

According to Wilson, the only time you would not use a high-altitude ball at elevation is if the temperature were to drop to the mid-40s or lower.  

In the case for Lee, who played college tennis at Rutgers University, but grew up in Albuquerque, which is around 5,300 feet above sea level, she would switch to a regular-altitude ball in the winter if she were outdoors to offset the low-rebound effect from the colder conditions. But if she were indoors, where the temperature is typically warmer, she would play with a high-altitude ball or, as Lee said, it would sail like 10 feet long.

To the human touch, the change in deformation isn’t noticeable. “But the minute a player hits with a ball that’s not suited for the environment, they’d understand the difference immediately,” Lee said.    

On the surface, subtle differences. But the reality is that these core changes can make a big difference.