won by current WTA players,

and you'll notice that nearly half

belong to just one person:

Serena Williams.

In fact, Serena is one of the most successful

tennis players of all time.

And that's in large part thanks to her iconic serve.

You can break Serena's serve into four parts.

The acceptance window is the area of the court

where the ball has to land in order for the serve to count.

It's determined by the minimum angle

for the ball to clear the net

and the maximum angle for it to land inbounds.

So, you can see here that the height

at which you hit the ball makes a big difference.

Chang Kee Jung: As taller you are, you have

more acceptance window.

Imagine Yao Ming learns to play tennis,

and if he hits the serve,

he will have a huge acceptance window.

Narrator: Serena isn't nearly as tall as Yao,

and she's not even among the tallest

female tennis players out there.

But she makes up for it.

Chang: So, Serena Williams does exceptionally well.

At the position when she hits,

her arms are fully stretched

and then she has a little bit of jump.

Narrator: The stretch and jump boost the height

at which she hits the ball,

increasing her acceptance window

and ultimately improving her chances

of hitting it inbounds.

Now, Serena isn't the only player who extends on the serve.

But, according to experts, she is much more consistent.

And it pays off.

Over the last 10 years,

she's won 82% of the games that she served.

That's a whopping 11% higher

than the average of the top 20 female players

over the same time frame.

But, of course, serving the ball inbounds

is just the first step.

What gets her all of those wins

is the fact that she makes her serves

extremely difficult to return.

That's where power and topspin come in.

Serena's power serve is one of the fastest in the game,

averaging 170 kilometers per hour.

That's about 12 kilometers per hour faster

than the average female professional tennis player.

Now, part of that speed is the result of pure strength.

Chang: Serena Williams, you can just see

she has, like, incredible, lean, muscular body,

so she can generate a huge amount of force.

Narrator: But what really sets Serena's power serve apart

is her timing.

To maximize power on the serve,

tennis players will bend their knees,

rotate and drop the hip,

lower the shoulder,

and, most importantly, twist at the trunk.

In other words, they use their whole body

to gain as much velocity behind the serve as possible.

But none of that matters if you don't hit the ball

at the exact moment when your arm is at full reach

and your body's in mid-swing.

In tennis, this is called fluidity.

And Serena is often credited

with the most fluid serve in the sport.

Chang: There will be no jerky motions.

You will smoothly hit the ball with maximum power.

Narrator: But there's a catch to these power serves.

Yes, they're fast, but the velocity

narrows the acceptance window,

making them less reliable.

That's why Serena also uses topspin to her advantage,

usually when her first power serve goes out of bounds.

Now, topspin serves are slower.

For example, Serena's average topspin serve

is about 145 kilometers per hour,

25 kilometers per hour slower than her power serve.

But topspin serves are more accurate

and still difficult to return.

That's because the topspin on the ball

forces it to drop sharply

right before it reaches the opponent.

That change in movement is thanks to a phenomenon

called the Magnus effect.

As the ball spins through the air,

it creates a pocket of lower pressure underneath it.

And it's that low pressure

that makes the ball drop abnormally fast,

catching many opponents off guard.

But Serena's second serve is not the fastest in the game.

Maria Sharapova, for example,

will fire her second serve at about 150 kilometers per hour.

But Sharapova sacrifices accuracy for speed here.

For comparison, Serena has scored, on average,

2 1/2 fewer double faults per match

than Sharapova over the last 10 years.

Chang: So, she combines the first serve,

of high speed, which is a little bit more risky,

but her second serve is safer.

However, it has a high topspin, and it's awkward to return.

And combining these two, she becomes excellent,

excellent service player.

Narrator: Meaning she scores more aces on average.

An ace is a successful serve

that the opponent's racket cannot reach

and therefore doesn't return.

Over the last 10 years,

Serena has recorded four more aces per match

than the average top 20 WTA players.

Last, but not least,

is the coup de grâce of Serena's serve:

disguise.

Chang: Another exceptional quality

is her ability to serve the ball

without telling the opponent

what kind of serve she will make.

Narrator: Check out this ball toss.

Normally, the way a player tosses the ball

will give their opponent a clue as to what serve is coming.

But not Serena.

She tosses the ball to the same spot,

with the same body position,

every single time.

Her toss is so consistent and smooth

that she manages something most others

find impossible to replicate.

She can disguise the serve.

So, Serena could be about to deliver a topspin serve,

a power serve, or even a slice serve.

Yet the opponent has no way of knowing what's coming.

Chang: These things are all disguised

until the very last moment.

And that makes Serena Williams'

serve exceptionally difficult to return.

So, for as long as Serena is competing,

these three words have the power to strike fear

into many pro players' hearts: