Serena Williams displays her power at the 2013 U.S. Open.
Maria Sharapova, ranked No. 3, withdrew from the tournament before it started. It was fitting because Sharapova no longer figures in the conversation as to who is No. 1.
Meanwhile, Flavia Pennetta, who had dropped out of the Top 50, reemerged to pull off a string of upsets.
Li Na re-stablished herself as a contender in Grand Slams.
The Grand Slams are over. The biggest prize left is the WTA Championships. Williams and Azarenka have locked up spots. Everyone else is in hot pursuit.
The latest WTA rankings are out and so are the post-U.S. Open power rankings.
Ekatarina Makarova in U.S. Open match against Li Na
Ekatarina Makarova moved up from No. 25 to No. 21 in the WTA rankings.
She makes No. 20 in the power rankings because of the way she played against top opponents at the U.S. Open.
Makarova defeated Bethanie Mattek-Sands, Sabine Lisicki and No. 4 Agnieszka Radwanksa, all in straight sets.
She pushed No. 5 Li Na to three sets.
A solid all-around player, Makarova is best known for upsetting Serena Williams at the 2012 Australian Open.
Sabine Lisicki reaches for the ball at 2013 U.S. Open.
Sabine Lisicki's booming serve and powerful groundstrokes seem tailor-made for the hard courts.
Yet the German struggles to duplicate her grass-court success at the U.S. Open.
Still, Lisicki remains in the top-20 power rankings because she has the power, movement and court presence to compete with the best in the game.
Flavia Pennetta enjoyed a great run at the 2013 U.S. Open.
Flavia Pennetta leap-frogged from No. 83 to No. 31 in the WTA rankings.
Once ranked in the Top 10 (2009), Pennetta fell out of the Top 50 after suffering a wrist injury last year.
She underwent wrist surgery. Finally, she seems to have recovered.
Pennetta surprised many by reaching the semifinals.
She defeated fellow Italian and former No. 4 Sara Errani and former U.S. Open champion Svetlana Kuznetsova en route to the semifinals.
Carla Suarez Navarro celebrates win at 2013 U.S. Open.
After she upset No. 8 seed Angelique Kerber, Carla Suarez Navarro thrilled the U.S. Open crowd with an uncharacteristic manic post-win celebration.
Although overwhelmed and outmatched against Serena Williams in the quarterfinals, Suarez Navarro took a huge step forward.
She finally added hard-court success to a career built on clay.
Kirsten Flipkens in first-round match at 2013 U.S. Open
Kirsten Flipkens followed up her Wimbledon semifinal appearance with a solid showing in Cincinnati.
In Cincy, she took out Venus Williams in the first round, but was beat easily by Serena Williams.
Flipkens appears to be playing with purpose. Her serve and volley game is a welcomed change of pace among all the baseline boomers.
Despite the first-round loss to Venus at the U.S. Open, Flipkens is having a good year.
This week, she is set to defend her title in Quebec.
Ana Ivanovic at the 2013 U.S. Open
Ana Ivanovic will forever be defined as the former No. 1 and winner of the 2008 French Open.
That's because that is where her career peaked.
Despite reaching the fourth round at the U.S. Open, Ivanovic continues to struggle with her serve.
Her game hasn't improved enough to make her a serious contender to win major tournaments.
Samantha Stosur during loss at 2013 U.S. Open
Despite being upset by teenager Victoria Duval, Samantha Stosur remains one of the top players on tour.
Winner of the 2011 U.S. Open, Stosur had an off night against Duval.
She fell from No. 11 to No. 18 in the WTA rankings. But she comes in at No. 14 in the power rankings.
One first-round upset loss doesn't change the fact that Stosur is one of the few women on tour who, when playing her best, can beat anybody.
Caroline Wozniacki goes for serve during 2013 U.S. Open.
Caroline Wozniacki comes in at unlucky No. 13.
The former No. 1 has produced uneven results this year.
Her performance at the U.S. Open was no different. She lost in the third round to No. 136-ranked Camila Giorgi.
Simona Halep at the 2013 U.S. Open
Outside of Serena Williams, Simona Halep is the hottest player on tour right now.
Halep, ranked No. 19, has won 20 of her last 23 matches. She's won four WTA titles this year.
During this amazing run, Halep has victories over Caroline Wozniacki, Marion Bartoli, Flipkens, Suarez Navarro, Stosur and Petra Kvitova.
Halep is one to watch.
Sloane Stephens during match against Serena Williams at 2013 U.S. Open
Throw out her first-round match at the U.S. Open and Sloane Stephens had an impressive run. She lost to eventual winner Serena Williams.
In a showdown between emerging American stars, Stephens dismantled Jamie Hampton.
Williams exposed Stephens' fear of charging the net.
Still, Stephens gave the No. 1 player all she could handle in the first set.
Stephens' game continues to improve. That's why she comes in at No. 11 in the power rankings.
Angelique Kerber at 2013 U.S. Open
Angelique Kerber has been a mystery this season.
The hard-hitting German is ranked No. 9. But she is No. 10 in the power rankings.
She is not playing with the confidence she displayed last season.
Last year, she reached the quarterfinals at the French Open and the semifinals at Wimbledon.
But recently, she has struggled.
Petra Kvitova in match at 2013 U.S. Open
Petra Kvitova continues to disappoint.
Winner of Wimbledon in 2011, Kvitova has Grand Slam champion talent. But she lacks the consistency and the fitness to compete with the best on tour.
Still young, she has time to get back on track. Right now, she's No. 9 mostly because of what she can do instead of what she's done lately.
Roberta Vinci volleys in a match at 2013 U.S. Open.
Kudos to Roberta Vinci for elevating herself from doubles specialist to serious singles player.
Like her doubles partner, Sara Errani, Vinci has found a way to break into the Top 10 without any discernible weapons.
Persistent and consistent, Vinci has reached the fourth round or better at each Grand Slam event.
She has won two titles this year.
She reached the quarterfinals at the U.S. Open for the second year in a row.
Sara Errani in match against Flavia Pennetta at 2013 U.S. Open
Sara Errani, the little engine that could, may be finally running out of steam.
She fell from No. 5 to No. 6 in the WTA rankings. She comes in at No. 7 in the power rankings because players are exploiting her weak serve. They have figured her out.
Hope she enjoyed her run in the Top Five. It's unlikely she's headed back.
Jelena Jankovic during match against Li Na in 2013 U.S. Open
Jelena Jankovic is finding her form again.
The former No. 1 has clawed her way back to the Top 10.
She moves ahead of Errani and Kvitova in the power rankings because she has been more consistent.
She has also rebuilt her game enough to challenge the likes of Serena Williams and Azarenka.
Agnieszka Radwanska eyes the ball at 2013 U.S. Open.
The good news for Agnieszka Radwanska is that she remains in the Top Five in the WTA rankings.
The bad news is that she appears stuck at No. 4.
She lands at No. 5 in the power rankings because, despite her inability to break up the "Big Three" (Williams, Azarenka and Sharapova), Radwanska is still one of the craftiest players on tour.
A "tough out," "Aggie" has replaced Wozniacki as the premier counterpuncher on tour.
Maria Sharapova in a loss to Sloan Stephens in Cincinnati
Going into the French Open, Sharapova was competing for No. 1.
Now she isn't even in the conversation.
She suffered an early exit at Wimbledon. She withdrew from Toronto and Stanford. She suffered a first-round loss at Cincinnati and then withdrew from the U.S. Open.
She has the third-most Grand Slams of all active players competing in singles. (Martina Hingis is competing in doubles only).
Li Na in her semifinal loss to Serena Williams at 2013 U.S. Open
Although Li Na is No. 5 in the WTA Rankings, she comes in at No. 3 in the power rankings.
Her run at the U.S. Open re-established her as a top-tier player.
She has improved her serve and play at the net.
She reached the final in Australia, the quarterfinals at Wimbledon and the semifinals at the Toronto, Cincinnati and the U.S. Open.
Right now, she is the third-most dangerous player on tour.
Victoria Azarenka in semifinal match in 2013 U.S. Open
After beating Serena Williams in Cincinnati, Azarenka created some doubt as to who had the edge going into the 2013 U.S. Open.
It was the second time she had defeated Williams this year.
We wondered, had Vika figured Serena out?
Still, Azarenka proved she is clearly the the top contender.
Serena Williams holds trophy after winning her fifth U.S. Open title.
Serena Williams' victory over Victoria Azarenka leaves no doubt as to who is No. 1.
She has won more Grand Slam titles than anyone else this season. She has won nine WTA titles.
She holds the No. 1 ranking by a 2,700-plus point margin. She leads the field in prize money earned.
As tough as Azarenka fought, it still wasn't enough to beat Williams, who was not playing at her best.
Going forward, Williams' toughest opponent will be Father Time. He will defeat her one day. The only question is how many more Grand Slam titles can she capture before he catches up with her?