they have to get in the zone.

Novak Djokovic will bounce the ball, first with his racket,

then with his hand.

Naomi Osaka will tap her thigh.

Rafael Nadal will pull his shorts,

the sleeves around his shoulders,

touch his nose,

and then finally move his hair to the back of his ears.

But there's one ritual almost every player does:

They're looking at the fluff.

Choosing a tennis ball with the perfect level of fluffiness

is a long-held tennis tradition.

And some players believe the right ball can help them win.

But does fluff actually make a difference?

In a professional tennis match,

six balls are used over nine games.

Before serving,

players typically ask the ball boy or girl for 3 or 4 balls,

and select one or two.

And most of these players are looking for something specific:

“I try to get the ball that is more new for the first serve.”

Daniil Medvedev is a professional tennis player

who's currently ranked Number 2 in the world.

He's defeated Novak Djokovic,

Rafael Nadal,

and Dominic Thiem.

He just competed in the Ultimate Tennis Showdown.

And the French Open.

And he cares about fluff.

“You try to find a ball that has less hair, actually,

because it will go faster in the air.

You don't want this big ball, you know,

that has a lot of humidity in it for the first serve,

because it's not going to go fast.”

Michael Kosta, a former professional tennis player,

does the same thing:

“I would always want the more compact,

tighter-felt ball for my first serve.

I felt like that moved faster through the air.

It gave me more confidence to hit that first serve,

and I would save the kind of juicier, bigger, fluffier ball

for my second serve, as almost a safety ball--

I need this big, slow, beach ball to get it in the court,

just in case I miss my first serve.”

“On the second serve they're looking for control more than speed.

My name is Patrick Mouratoglou.

I'm a professional tennis coach.

I'm working with Serena Williams, For the last, almost ten years.”

Yes, the Serena Williams.

“When the ball is fast, you have less control.

So on the second serve, they don't want to hit a double fault.

And the fluffier the ball, the more grip you have.”

“It's the kind of tactic that, I should say, doesn't affect so much.

Like, you're not going to win the match just because you chose a fluffier ball.”

But there is some science behind this ritual.

So, Rabi, how involved, or how much information do you know about tennis?

“Oh, I think I know enough to be dangerous.

In terms of the science.

I'm a pretty lousy tennis player.”

Dr. Rabindra Mehta works for the NASA Ames Research Center,

and has been studying the aerodynamics of sports balls

for more than four decades.

“The fluff is the whole key.

With tennis, you get what we call 'extra drag.'

Drag is the force that slows the ball down

as it's flying through the air.

Initially, when you pull it out of a can,

you'll find it's pretty compact, and so it has one level of drag on it.

But when you start playing with it,

it tends to fluff up, initially, the first few games,

and the drag actually goes up.

Like the hair on our arms, it'll contribute to the drag.”

That's why swimmers have their own ritual

of shaving all their body hair before big meets:

to make it easier for them to move through the water.

To figure out how much the fluffiness of a ball

actually affects its speed,

Dr. Mehta and his team used a wind tunnel.

“We had the ball on the balance, so we could measure

how much drag force, how does it change?”

They compared tennis balls with different amounts of wear,

and proved that the fuzzier the ball, the slower it moves.

“And the way we proved that was just by literally shaving the ball,

to try and get it to match the other balls.

And we were able to do that.”

On the court, this drag can mean

that if a player serves with a fluffier ball,

the person receiving gets slightly more time to react.

And the server then gets slightly more time to return it.

“I was at the US Open, and we had some people there

who were interested in what we were doing, and they actually sent us balls

that had been used in the actual tournament.

So when we started looking at those balls, it was pretty obvious

that they were preferentially using two or three balls out of the six.

So it wasn't like there was equal wear.”

Beyond the science, the ball selection still serves a purpose:

“If I believe, when I'm match point down, and I have to hit a second serve,

that my rituals, of finding that bigger, juicier, feltier ball,

are going to give me a tiny little bit of safety on the second serve,

that little bit of confidence can be what helps you eke out the match.”

“There is just such an incredible amount of pressure and stress on them.

So the fact that it makes them feel better, makes them play better”

That's why routines are such a big part of this sport:

“The goal of the routine is to refocus for the next point.

And it becomes a habit.”

Some routines can be more obsessive than others,

like Nadal meticulously lining up his water bottles on the sidelines.

But for many, that little ritual of picking the right ball is enough.

“Most of the players choose their balls.

Serena doesn't, by the way.

She's one of the only players who does not.

She's taking only one ball to serve, one by one.

So if she misses her first serve, then she asks for a second ball --

where most other players take two balls, keep one in their pocket…

And she takes the first ball that comes.

She never asks for another ball.”