In the summer of 2010, Novak Djokovic faced a crossroads. Long regarded as an up-and-comer, the charismatic young Serb, still just 23, seemed to be losing his upward trajectory. It was hard to think of him as anything but a victim of timing, a member of the "If only he had come along when Federer and Nadal weren't at their peaks" class that included players like Tomas Berdych and, eventually, Andy Murray. Between 2007 and 2010, I'm not sure I ever thought about him without a wistful, almost regretful expression.

By the time he was turning 21, Djokovic was clearly one of the three best tennis players in the world. In the 16 slams between 2007 and 2010, he reached 13 quarterfinals and eight semis. He won the 2008 Australian Open and showed prowess on all courts, making at least one semifinal at both the French Open and Wimbledon.

The problem: he was by far the third-best player in the world. His record at slams in 2007-08 was 37-7 -- 1-6 against Roger Federer and Rafael Nadal and 36-1 against everybody else.

By 2009, it seemed Djokovic might be losing the plot a bit. He was forced to retire from heat exhaustion against Andy Roddick in the Aussie quarterfinals. He lost to Philipp Kohlschreiber at the French Open and Tommy Haas at Wimbledon. He reached the U.S. Open semifinals again, but the following year the downward trend held. He lost in five sets to Jo-Wilfried Tsonga in the Aussie quarterfinals, then in five sets to Jürgen Melzer in the French quarters.

At Wimbledon that summer, he got a lifeline. He reached the Wimbledon semis again, and Federer had been upset by Berdych in the quarterfinals. But Berdych blitzed him as well. He was falling victim to both iffy fitness and blown opportunities.

Again, Djokovic was still only 23 years old and was clearly an elite tennis player. Thanks to his conquest of Melbourne in January 2008, he would be forever known as a slam champion, and while professional tennis struggles to take appropriate care of a large number of its pros, it goes without saying that its top 10 do quite well from the perspective of prize money and endorsements.

Novak Djokovic's ability to run and hit a tennis ball, along with his charm and marketability, had set him up for life. But he decided to see if he could do more.

Three things happened in the latter months of 2010 that turned his career around, both literally and metaphorically.

1. He led Serbia to a Davis Cup title. His dominant victories over Gilles Simon and Gael Monfils set the table for a 3-2 win over France in the finals. Serbia had never made even the quarterfinals before 2010, but he played brilliantly all year for his country, and he was responsible for the ultimate breakthrough that December.

2. He got in the best shape of his, and anybody else's, life. Professional tennis players are world-class athletes, obviously, and everybody works hard. It's difficult to stand out when it comes to fitness, but he changed his diet because of a gluten sensitivity and rededicated himself to yoga and flexibility, and he went from suspect to untouchable in the fitness department.

3. He hit these three forehands.

For the last five years, Undead Djokovic has been the scariest player in tennis. When he gets a taste of his own blood, his level of play seems to rise dramatically. The 2010 U.S. Open semifinals, in which he faced two match points against Federer and hit three consecutive murderous forehands, was the birth of the undead. He ultimately lost to Nadal in the finals, but that was it for a while -- he beat Nadal in six different finals in 2011, including Wimbledon and their U.S. Open rematch.

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Djokovic showed up in Australia that following January in a new place, mentally and physically. He won three slams in 2011 and has won another five since. Furthermore, since his loss to Melzer at Roland Garros in 2010, he has made at least the semifinals of 20 of the last 21 slams.

His rise also changed the narrative. Instead of "How many slams could Djokovic have won without Federer and Nadal around," it is now, "How many would they have won without him?" Since 2011, he has gone 8-6 against the two in slams. He has beaten Nadal in three finals and a quarterfinal; he has beaten Federer in the last two Wimbledon finals and in three semifinals. As the two all-time greats battle the inconsistency that comes with age and, in Nadal's case, injury, Djokovic is ever-present. He doesn't always beat them, but he's always there, the fittest, most grueling roadblock in tennis. Without Djokovic, Federer might have 20 titles. Nadal might have 17. They already hold the top two spots on the all-time slam titles list (Federer is in first place with 17, Nadal is tied with Pete Sampras with 14), and they could have far more.

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Novak Djokovic now has nine slam titles, more than Andre Agassi, or Jimmy Connors, or John McEnroe, or Stefan Edberg, or his coach, Boris Becker. In tennis' modern era, only four players have won more. He will probably tie Björn Borg's 11 by 2016. He is now the Big 1 of the ATP Tour, and if Stan Wawrinka hadn't played the match of his life in the French Open finals a little more than a month ago, Serena Williams wouldn't be the only singles player going for a 2015 grand slam at next month's U.S. Open.

Djokovic is everything you would want a champion to be. He is graceful and erudite, charismatic and self-effacing. On the court, he is both faces of a comedian -- the person who can make you laugh, and the tortured soul whose failures and insecurities are unhidden. He curses himself on the court, then plays better. He takes an opponent's best shot, then hits a better one. He is still only 28, and while he has dealt with back and wrist injuries at times, his injury track record is as good as anyone's. He could have another few years left at this level.

Federer showed us all how high the ceiling could be for men's tennis. Nadal accepted the challenge and topped him, and through tenacity, an obsession with fitness and improvement, and a taste of his own blood, Djokovic raised the bar even higher.

One has to wonder where women's tennis could have gone if more had taken Serena Williams' challenge. If only it were that easy.

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"It’s our decision to keep her as the smallest player in the top 10," said Tomasz Wiktorowski, the coach of Agnieszka Radwanska, who is listed at 5 feet 8 and 123 pounds. "Because, first of all she’s a woman, and she wants to be a woman."

Last week, the New York Times' Ben Rothenberg penned a piece about the role of body image in women's tennis and how it might tamp down a player's ambition when it comes to matching the runaway success of Serena Williams. It is a complicated, fascinating, and in the end rather depressing reminder of how there is always another story line at play when female athletes are discussed. It is especially complicated in tennis, a sport dominated by individuals.

At this point, Williams gets serious mention for any Best Female Athletes Ever discussion. Her career has seen three different peaks, any of which would have defined a player as one of tennis' greats.

In 2002-03, she went on a streak of winning five of six slams, including four in a row, the "Serena Slam," from the 2002 French Open to the 2003 Aussie Open. In 2008, she emerged from a lengthy battle with injuries and personal issues and caught fire again. She reached the 2008 Wimbledon finals (where she lost to her sister Venus), then ripped off five titles in eight slams.

Again, injuries held her back. She stepped on broken glass late in 2010, then, while recovering, she suffered a hematoma and pulmonary embolism. (A pulmonary embolism is a nightmare. Someone close to me dealt with an embolism a few years ago, and it took her two full years to feel like she was at full strength; now, seven years later, she still deals with scar tissue behind her legs, where the effects were greatest.)

By the end of 2011, however, she was back in the U.S. Open finals. At Wimbledon 2012, she won her 14th career slam; she's now won eight of the last 13. Since losing to Virginie Razzano in the first round of the 2012 French Open, she's 69-5 at slams. That's 14 wins for every loss. This is her most sustained level of greatness. She's 33 years old.

33 years old. Martina Navratilova, the poster child for longevity, won one slam after she turned 31. Chris Evert won two after her 30th birthday. The last of Steffi Graf's 22 slam titles came right before she turned 30. And Serena has won four in a row. We've never before seen what Serena is doing.

Many, however, have ascribed an asterisk to her record. Her level of competition just isn't the same, you see. The women's field is weak. Her 21 slam titles are therefore less impressive than Martina's or Chrissie's 18. That's a pretty neat trick, really -- if you dominate your competition too much, it hurts your résumé. That was a knock against Roger Federer, too, until Nadal and Djokovic came along.

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Of course, on the list of Serena critiques, that one ranks pretty low. She's dealt with an appalling level of criticism, and it began when she was a teenager. Her father, Richard, was accused of fixing matches between Serena and Venus. She dealt with countless examples of racist comments. Her intensity was viewed as a sign of unwomanly behavior, her build a sign of manliness, her withdrawal from the public a sign of laziness. She has forever been a square peg in a sport that prefers its round holes.

Such is the price of being different in the most individual of sports. We find your every flaw in tennis, and then we invent more.

It is perhaps no wonder, then, why so few women have attempted to approximate Serena's strength, to the extent that they even could. There is upside, sure, but is it worth the risk? In taking his fitness to a new level, Novak Djokovic became leaner, more nimble, and actually even more marketable.

But what's the draw for, say, Aga Radwanska? If she were to put on 15 pounds of muscle, maybe she could find the offensive game to finally and permanently break through. The 26-year old has reached four slam semifinals and one final (Wimbledon 2012, where she lost to Serena in a gripping three-setter) because of her speed and an elite defense that earned her the "ninja" moniker. Maybe some extra oomph on her serve and forehand would make all the difference in the world. And maybe that new level of success would usher in a new level of endorsements and cash.

Or maybe it would bog her down, take away some of her speed, and leave her without a clear identity, all while making her less attractive/marketable. The former is a risk for both genders, the latter a risk for only one.

(And to her credit, she said it far more gracefully than her coach did in that New York Times piece: "Of course I care about that as well, because I’m a girl. But I also have the genes where I don’t know what I have to do to get bigger, because it’s just not going anywhere.")

It takes more than simple strength to beat Serena, whose mental fortitude and tactical flexibility saw her through many crises at both Paris and London this summer. But those who do beat Serena are, more often than not, those who can bang. And some are absolutely trying to keep up in this regard. Victoria Azarenka has taken Serena to the brink quite a few times, including in this year's Wimbledon quarterfinals. Petra Kvitova's strength was the basis of two Wimbledon titles. Sabine Lisicki took Serena down at Wimbledon in 2013 and boasts one of the only serves on tour as powerful as hers. Sam Stosur, who knocked Serena around in the 2011 U.S. Open finals, has a physique carved out of granite. And it goes without saying that 21-year old Wimbledon finalist Garbiñe Muguruza, now the No. 9 player in the world, has some of the strongest, most hypnotic ground strokes you'll ever see.

But for those with leaner body types -- Maria Sharapova, Ana Ivanovic, Aga Radwanska, Caroline Wozniacki -- there is a risk-reward balance that it appears female athletes will always struggle to overcome. In aiming for the sky, you risk your bottom line.

Late-career Serena, for those who have taken the time to notice, is a joy. She's making friends. She's attending Taylor Swift concerts. She's hocking goods on Home Shopping Network. She has established an almost impossible combination of grace and uniqueness. But it came with trial and error. She has raged at officials and stared daggers into reporters. She has withdrawn from public life.

She is proof that you can survive the battle but that you also can't avoid it. Women's tennis hasn't produced a Nadal or Djokovic to her Federer, and if she can remain healthy, she might continue to dominate until it does.