Is it the power or the placement? The variety or the velocity?
What we do know is this: Serena Williams is going for her 20th Grand Slam title this week at the French Open. One shot, more than any other, is responsible for her invasion of the history books. Twenty of the game's greats dissect her epic serve.
"That's the biggest weapon out there, for sure. I think that's the biggest weapon there has ever been in the sport. Her serve is phenomenal. ... She has so much force, strength. And the variety -- even if it's a little off -- her first serve and kick serve is tremendous."
-- Steffi Graf, owner of 22 Grand Slam titles
"Her serve is amazing. It's amazing what an easy pace she has on it. You either have that, or you don't. But she seems to do it quite consistently, which is not an easy thing because when she does go full blast, you can spray it, but she doesn't tend to do that very often. Plus, she has a solid second serve, so that, in the women's game, is obviously huge to have, and it's a great advantage for her."
-- Roger Federer, 17-time Grand Slam champion
"Normally, you can see where the player is serving, or you can start seeing a pattern. But Serena is so good at mixing it up. And even sometimes when you see it, it comes so fast that even when you're there, it's hard to return it back."
-- Caroline Wozniacki, No. 5 player in the world
"First of all, she has a very powerful serve. It's a bomb. It's also very hard to read where she's serving, so you guess or you're just lucky because when she's serving, you just have no idea where the ball is going to go."
--Agnieszka Radwanska, 2012 Wimbledon runner-up to Serena Williams
"It's not that she serves only fast. She just varies the speed and the slice and the spin, and she's so exact on the spots. She just keeps you guessing and on your toes because you don't know what to expect. There's none comparable."
-- Andrea Petkovic, No. 10 player in the world
"You know, it's the snap at the top and the way she can snap that ball with her wrist and her hand at the top. It's like throwing a football. ... If you look, a lot of the women's serves are very forced, and a lot them use their arms, and they're muscling it a lot. But she's free-flowing and snaps it with her wrist at the top."
Chris Evert, 18-time Grand Slam champion
"I think the mechanics are excellent -- among the best. And what makes her mechanics good are really all the moving parts working in concert. ... One of the great things about her serve is the toss is almost always where she intends it to be. In this day and age, if you think about all the women who have errant tosses, that's a huge advantage, mentally, to know your left hand and ball toss is going to be on mark."
-- Tennis analyst and former WTA Tour player Pam Shriver
"Even early in her career, it was evident that Serena had an excellent, already intimidating and technically sound serve. ... Like her game as a whole, it will go down as one of the greatest, if not the greatest, serves of all time."
-- Annie Miller Borus, first person to beat Serena Williams in a WTA Tour match
"Serena's serve is so incredibly technically sound. She can power right through her opponents or place it on a dime when she's really feeling it."
-- John Isner, occasional mixed doubles partner of Serena Williams
"I played her, and I had match point, and she aced me on that match point, and I barely saw the ball. ... She is one of the biggest servers, but it's not about serving well -- it's all about serving well on the key points, and you cannot compare that. That's her biggest advantage."
-- Svetlana Kuznetsova, two-time Grand Slam champion
"I'm racking my brain ... Venus had a really good first serve, but there has never been a woman with such a dominant serve. Lots of guys have had great serves. Martina [Navratilova] had a great slice serve, but never was a woman this dominant. ... The reason Serena is going to be able to play for a while is that shot is still by far the most dangerous weapon in the game, the most reliable in the game and the only one you control."
-- Brad Gilbert, tennis analyst
"Her biggest strength is her serve. Maybe it's something that has saved her in many matches, situations where you cannot get the racket on the ball."
-- Maria Sharapova, No. 2 player in the world
"I think her technique is perfect, so when she throws the ball, you don't know if you have to go to the right or go to the left. ... And then, obviously, the power -- it goes like 200 kilometers. If you put these two things together, it's hard."
Garbine Muguruza, winner over Serena Williams at the 2014 French Open
"I think the disguise is what makes it so great. You really don't know from the same ball toss which way she's going to go. No, I never faced any better serve in my career."
-- Mary Joe Fernandez, Fed Cup captain, tennis analyst and former player
"She is very strong but at the same time very flexible. The combination of these two aspects enables her to dominate with [her] serve."
-- Max Mirnyi, two-time Grand Slam mixed doubles champion with Serena Williams
"I think she has good technique and also the power. Of course, her placement is very good, and she knows to change many angles when she serves. I think she's the toughest serve because she has power and placement and good technique, and she has a high percentage with her first serve."
--Simona Halep, 2014 French Open runner-up and No. 3 player in the world
"I think she can put 10 serves in a row in one spot, and that's the biggest weapon she has to me. She can hit it anywhere, at any angle, and wherever she wants to put it."
-- Karolina Pliskova, 2015 WTA ace leader with 238
"Besides having more power than the other women, Serena has great location and variety on her serve. She can accurately hit all the spots with many different spins."
-- Bob Bryan, American doubles legend
"It's the best women's serve. And overall, it's one of the biggest weapons for anyone, male or female. It definitely bails her out more than any other shot and more than other people's shots. Like Roger Federer or Nadal's forehand, it sets up the point right away."
-- Martina Navratilova, 18-time Grand Slam champion
"Yeah, I could return my serve because I actually like it when people serve hard to me. I like it when people serve soft too, but I really like when I get a hard serve."
Serena Williams on Serena Williams