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.