Novak Djokovic and Rafael Nadal during 2013 U.S. Open trophy ceremony.
What a difference a year makes.
Last year, the great Roger Federer won his 17th Grand Slam title. This year, he dropped out of the Top Five.
Andy Murray became a national hero. Novak Djokovic looked less than invincible, and Rafael Nadal looked unstoppable.
Yet only Nadal and Williams will finish the year with more than one Grand Slam title. They also seem likely to finish the calendar year ranked No. 1.
Although there is plenty of tennis left to play in 2013, the Grand Slams are finished.
Let's face it, Grand Slams define careers.
How the top stars perform in Slams determines how the public perceives them.
So how did their performances in 2013 impact public perception?
Li Na at the U.S. Open
Li Na cashed in on her 2011 French Open title to the tune of $42 million in endorsements.
In 2012 she appeared to be headed for a skid similar to that of 2008 French Open winner, Ana Ivanovic.
It looked like Li would be another one-season wonder.
But this year she retooled her game. She's back in the Top Five and a long way from being a has been.
Roger Federer in loss at U.S. Open
When Federer won the 2012 Wimbledon, he appeared to cement his status as front-runner in the "Greatest of All Time" debate.
This year, Federer failed to reach the finals in any of the Grand Slams.
Worse, he appears to have hit a wall. He's losing in earlier rounds to players ranked outside of the Top 20.
Federer could retire today and still would be considered by many as the greatest of all time.
But in 2013, Federer went from definitive "G.O.A.T" to tentative, as in we'll have to wait and see.
Maria Sharapova in loss at Cincinnati
Sharapova entered 2013 with a real shot at the No. 1 ranking.
She won Indian Wells and reached the finals in Miami. She also reached the finals at the French Open, where she lost to Williams.
Then things began to unravel.
Sharapova chose to use her Wimbledon pre-tournament press conference to air Williams' dirty laundry.
She complained about the slippery grass at Wimbledon before losing in the second round.
She fired her coach. Then she hired Jimmy Connors. She fired Connors after one match, a loss to Sloane Stephens in Cincinnati.
Then Sharapova reportedly filed a petition to change her name to Sugarpova, to promote her candy company.
She withdrew from the U.S. Open.
Her career seems headed in the wrong direction. She's still young enough to rewrite her tennis story.
But for someone so image conscious, Sharapova went off the rails this year.
Andy Murray hoists trophy at 2013 Wimbledon.
Andy Murray tossed the monkey off his back and replaced it with a super hero's cape.
Murray delivered Great Britain it's first home-grown Wimbledon champion in 77 years.
Last year, Murray won a gold medal at the 2012 Olympics in London and the U.S. Open.
But he fell short at Wimbledon. His loss to Federer left the Scot in tears.
His triumph at Wimbledon this year freed Murray to concentrate on being the best, not just the best Brit.
Victoria Azarenka in 2013 U.S. Open final against Serena Williams.
Victoria Azarenka created some distance between herself and the rest of the challengers to Serena Williams.
There is still a gap between her and Williams. But it's smaller.
For a while Azarenka's record against Williams mirrored that of Sharapova's.
That was until this year. Azarenka beat Williams twice this year.
Azarenka proved that she can hang with and even beat the woman who many believe is the greatest of all time.
A frustrated Novak Djokovic during 2013 U.S. Open final.
Djokovic started 2013 off ranked No. 1. He won the Australian Open and then set his sights on the French Open. The French Open is the only Grand Slam title that has eluded him.
This year, he lost in the semifinals to Nadal.
With so much focus on winning the French, Djokovic has ignored the fact that four of his six Grand Slam titles have come at the Aussie Open. And that five of the six have come on hard courts.
In his failure to defeat the "clay-court specialist" at the French Open, Djokovic has looked less than invincible.
Meanwhile, Nadal dominated on hard courts.
Serena Williams posing with 2013 U.S. Open championship trophy.
When Serena Williams arrived at the Swedish Open, tournament officials promoted her appearance like she was a rock star.
They released a video showing Williams stepping off of a private jet to the sound of Lady GaGa's hit "Paparazzi."
Williams, often a polarizing figure on tour, crossed the threshold from iconic tennis player to international star.
She wowed crowds in Rome by speaking Italian. She spoke French for many of her post-match interviews at the French Open.
Her appearance in the 2013 U.S. Open final was a ratings bonanza. Despite going head-to-head with the NFL, the U.S. Open women's final outdrew the men's final.
Her player's box was filled with celebrities.
After winning the U.S. Open, Williams appeared on CBS This Morning, Live with Kelly and Michael and Good Morning America. She was a guest on the Late Show with David Letterman just before the U.S. Open.
As she continues to chase Grand Slam titles, Williams has reached rock star status.
In her latest commercial, a Radio Shack ad, Williams fires flames like a 1980s rock guitarist.
Rafael Nadal holds U.S. Open trophy in New York.
What a difference a seven-month absence makes.
Last year, after Nadal lost at Wimbledon and withdrew from the Olympics, his future was in doubt.
Tennis fans wondered if he'd be the same explosive Rafa post knee rehab. He wasn't.
He was better.
Now Nadal is challenging Federer in the greatest of all time debate.
He seems like a lock to pass Federer in Grand Slam titles.
His amazing post-knee injury run includes French Open and U.S. Open titles. He also won at Indian Wells.
Juxtaposed to Federer's decline, Rafa's rise seems meteoric.
Nadal's potential to be the greatest has been there for a while. But we thought any real discussion would happen long after Federer retired.
It's unclear if Federer has another Grand Slam championship in him. If he doesn't, Rafa could catch Federer before he retires.