Li Na celebrates her quarterfinals win at 2013 US Open.
But there is one other player left in the tournament who, like Azarenka, has the power and championship pedigree to derail Williams.
It's Li Na.
With so much attention on Azarenka and Williams, Li has been slightly overlooked.
Ranked No. 5, Li is having a sensational year.
She is the first Chinese player to reach the semifinals at the U.S. Open.
She also reached the finals at the Australian Open where she lost to Azarenka, 6-4, 4-6, 3-6. It was a bizarre match that saw Li evaluated for a possible concussion.
It was Li's second Australian Open final in three years and it solidified her standing as a top-notch hard-court player.
Yet she enters this semifinal match as an underdog.
Anyone who plays against Williams is an underdog.
With 16 Grand Slam titles, including four U.S. Opens, Williams also has all the pressure. It's all or nothing for Williams. She must repeat as champion or her stellar season will be considered a disappointment.
A possible rematch with Azarenka lurks in the future. However, Williams must proceed with caution. She can't look ahead or underestimate the talented Li.
Besides the showdown with Sloane Stephens, this is the most anticipated women's match so far.
Serena Williams and Li Na at WTA Championships.
Serena Williams leads the head-to-head matchup 8-1.
Li's only win against Williams came in 2008 in Stuttgart, Germany.
The matches appear to be getting closer, but Williams continues to prevail.
Li Na reaches for ball in quarterfinal match at 2013 US Open.
Li Na is making her first appearance in the U.S. Open semifinals.
She arrives with some new wrinkles to her game, including improved net play.
Her serve continues to be the weakest link. She had eight double faults in her quarterfinal match after she only had 10 in the previous four matches combined. She also had 42 unforced errors in that match.
Her first test came against British upstart Laura Robson. Li held off the hard-hitting Robson, 6-3, 7-5, as she won all nine of her net points.
Li played her best match against former No. 1 and 2008 US Open finalist Jelena Jankovic. Against Jankovic, Li won 77 percent of her first serve points. She also won all seven of her break-point chances and 83 percent of her net points.
Her play at the net is much improved while her approach shots look smoother and more fluid.
Although her temper occasionally flares up in frustration, Li has remained relatively calm throughout this year's U.S. Open.
Serena Williams goes for a forehand in her quarterfinal match at US Open.
Serena Williams dispatched of her quarterfinal opponent, Carla Suarez Navarro, 6-0. 6-0, in less than an hour.
The key match for Williams, who has yet to drop a set, came against Sloane Stephens. Stephens matched the power and speed that Williams will see from Li.
In that match, Williams showcased her shot variety and selection. While Stephens kept trying to pound the ball into winners, Williams unleashed drop shots, lobs, backhands and forehands down the line and ridiculous cross-court winners.
Surprisingly, what Williams hasn't shown off is her trademark ace serve. Williams only has 21 aces in the tournament so far. When she won Wimbledon in 2012, she had 102 aces.
While her serve remains the best in the women's game, it hasn't been the lethal weapon it used to be.
Li Na's coach, Carlos Rodriguez, sits behind her husband at the 2013 French Open.
Serena Williams' relationship with her coach, Patrick Mouratoglou, grabs headlines.
However, it's Li's relationship with her coach, Carlos Rodriguez, that could be an X-factor.
During the same period that Williams started working with Mouratoglou, Li began working with Rodriguez.
Rodriguez was the long-time coach of Justin Henin, a player with similar size to Li. Henin, although only 5'6", was able to handle big hitters. Henin has a 6-8 record against Williams, but four of those victories came on clay.
Still, Rodriguez has helped Li bring some variety to her game. Li said that she feels more secure when Rodriguez travels with her.
Another X-factor could be the 3.6 million reasons Williams has to win the title.
Because Williams won the U.S. Open series this summer, she could would receive a $1 million bonus on top of her championship check.
For Williams, it's also about mood and attitude. Which Serena will show up to play? Will we see the pouting, agitated and frustrated error-prone Serena? Or will the dialed-in, ace master show up? If it's the latter, it's lights out for Li.
Anything less, and the new and improved Li could pull off the upset.
Li Na prepares to serve in her quarterfinal match at 2013 US Open.
Li Na has a solid all-court game.
Forget all the talk about her serve and volley. That style may look like target practice for Williams.
She can get into trouble with Williams if she struggles with her serve.
Oddly, Li has served more aces (29-21) than Williams has in this tournament. The problem is that she has also served twice as many doubles faults (18-9) as Williams.
If Li can hold her serve and put pressure on Williams' second serve, she can neutralize the huge edge her opponent has in the service game.
Serena Williams challenges a call in her match again Sloane Stephens at 2013 US Open.
Serena Williams is nearly impossible to beat when her serve is on because her serve ignites the rest of her game.
Although her serve has improved with each match, it's still not the master-blaster it was last year.
If she serves well, keeps her composure and hits twice as many winners as errors, Williams will find herself playing for her second consecutive U.S. Open title.
Serena Williams celebrates a win over Sloane Stephens at 2013 US Open.
Although their matches have gotten tighter, Serena Williams maintains a mental edge over Li Na.
The match will be close, as usual and Li will probably even break a few of Williams' service games. However, Li has a propensity to get tight in big moments.
Besides, Williams knows Li too well. A few new wrinkles won't be enough to overcome Williams' superior talent, mental toughness and championship aptitude.
There's a reason Williams enjoys a ridiculous record against Top-10 players because he gets up for the big matches. She also studies her opponents better than anyone in the game.
Williams also plays well after setbacks. She would like to make her surprising Wimbledon loss a mere speed bump in her tennis legacy.
Expect Williams to be focused, determined and victorious.