The 2024 U.S. Open women's tournament is down to its final four contestants and, for the second straight year, will include two Americans, first-time Major semifinalists Jessica Pegula and Emma Navarro.

23-year-old Emma Navarro will face the toughest task in all of women's tennis: a semifinal matchup with No. 2 seed Aryna Sabalenka, who has won two of the three most recent hard-court Grand Slams.

Meanwhile, for the second time in as many weeks, Pegula will face Karolina Muchova.

The two played on August 16 in the second round of the Cincinnati Open, where Pegula defeated the Czech opponent in a hard-fought three-set battle.

Here are the top storylines to know for each match.

Aryna Sabalenka vs. Emma Navarro

As impressive as Emma Navarro has been, she is about to face the biggest challenge of her career: defeating Aryna Sabalenka on a hard-court surface.

The 23-year-old American has actually already done so once (2024 Indian Wells) and is playing extraordinary tennis as of late, with impressive victories against Coco Gauff and most recently Paula Badosa.

Sabalenka, though, has performed even better.

In fact, since losing on August 10 to Amanda Anismova, the two-time Grand Slam champion has won ten straight matches while dropping just one set.

The No. 2 seed has consistently identified early weaknesses in opponents' games, often exploiting their weaker second serves.

Although Navarro has the strength to challenge Sabalenka, given the 26-year-old's form, it'll take a flawless performance to achieve what would be considered a massive upset.

Jessica Pegula vs. Karolina Muchova

In her quarterfinal match against Iga Swiatek, Jessica Pegula finally slayed the dragon, reaching her first major semifinal after six failed attempts.

The 30-year-old American has been otherwordly in the summer hard-court swing, winning 14 of her last 15 matches.

Pegula has yet to drops a set in the Open, though neither has her semifinal opponent, Karolina Muchova.

The two will face off for the second time in as many weeks, and though Pegula grabbed the latest win in their head-to-head (tied 1-1), Muchova has earned some high-quality wins in her 2024 U.S. Open campaign.

Notably, she defeated two-time U.S. Open champion Naomi Osaka in straight sets and then dominated Jasmine Paolini, who had reached back-to-back Grand Slam finals prior to the U.S. Open.

Muchova's top-tier consistency and athleticism are more than enough to challenge Pegula.

The smallest margin of error may be the most important in this matchup of two players performing similarly.