Reviewed by Tom Perrotta

Novak Djokovic, the No. 2 seed, who won his 38th-straight match of 2011 on Monday, called the balls "very, very fast" and "really difficult to control." Men's No. 3-seed Roger Federer, a 16-time Grand Slam winner, said they're "faster, indeed," especially when they're fresh. "That will be an issue," he said.

Samantha Stosur, last year's women's finalist, said that she thinks the new models are "a little bit harder."

A faster, harder ball, the thinking goes, could add a little zip to a serve or forehand, making it more difficult to return. "Maybe it's going to favor the servers and the big hitters," Djokovic said.

Mark Woodforde, the former doubles champ from Australia, peeled open a can of the new balls at a restaurant here Sunday evening. "Feel that," he said, squeezing the ball and indicating how firm it was. "They fly off the racket, and after you've played a bunch of games with them, they still don't seem to have much clay on them."

Babolat and the French Tennis Federation say the new balls have the same performance characteristics as the old ones. "The laboratory tests prove that these balls have the exact same specifications as last year," said Christophe Hayaux du Tilly, the federation's sponsorship manager. "The same speed, the same rebound, the same size, all the same."

Of the four Grand Slams, the French Open has changed ball brands the most recently. It used Dunlop before 2001, Technifibre from 2001 to 2005, and Dunlop again from 2006 to 2010. Wimbledon has stayed with Slazengers since 1902—though it did change to yellow from white in 1986—and the U.S. Open has used Wilson since the 1970s. The Australian Open also uses Wilson, since 2006. Babolat's contract with the French Open is for five years.

The change itself seems to have inspired more complaints than the physics of the new ball. The clay-court tournaments leading up to the French Open used Dunlop balls and the players would rather not have to adapt to new ones for the year's most important clay event. "That, for us, is the most frustrating part," Federer said.

Babolat, which began as a string company in 1875 and started making balls in 2001, had just a few months to design the ball. It subjected it to a barrage of lab tests and gave samples to the French federation and to players—and not just Babolat players, said Jean-Christophe Verborg, the company's international tour manager. The federation also sent each of the Top 10 players on the men's and women's tours a box of the new balls this spring. Player travel being what it is, though, it seems the balls mostly gathered dust. Stosur's box was sent to Liechtenstein, where she stayed for a bit during the European clay-court swing.

"I was only there twice; practiced one time each," Stosur said.

David Ferrer, a top-10 player from Spain who reached the finals in Monte Carlo and Barcelona, looked miffed when asked about his delivered balls.

"I didn't see them," he said.

The bulk of Babolat's player tests were conducted in Europe last fall. Players were given several unmarked balls and allowed to hit with them for as long as they liked, Verborg said. He was present for some of the tests, including Rafael Nadal's, which took place in October in Manacor, Majorca. (Both Nadal and Stosur use Babolat rackets.)

"They didn't see a difference," Verborg said. Verborg and Hayaux du Tilly suggested other factors might account for the players' perceptions. One possibility: Paris has had warm and dry weather of late, so the courts are harder, which can make them faster.

Kai Nitsche, vice president of racket sports at Dunlop, said laboratory tests can't tell you everything about a ball—especially how it feels.

"There really would have been no way to duplicate our ball unless they had our formula," Nitsche said. "The core itself, while natural rubber is the main component, has 14 additional ingredients." Nitsche politely declined to reveal the ingredients.

"We've been making balls for over 100 years," he said. "That's certainly not something we're going to give away."

The International Tennis Federation regulates tennis balls and tests them for compression, mass, size, deformation, rebound, and durability. But as Jamie Capel-Davies of the ITF's technical department explained, the ITF doesn't analyze composition.

"You don't even need traditional rubber, if you have discovered something else," he said. "Our test is a performance test rather than a materials test. We see if it looks and behaves like a tennis ball, not how you achieved it."

Hayaux du Tilly says he isn't bothered if the players say the ball is different, as long as they don't say it's defective.

"The important point is, no one said it is bad," he said. "All the players say this is a very good ball."

Ferrer agreed. "It's faster than the Dunlop, but I have no problem," he said.