Sloane Stephens has beaten Venus Williams in their only encounter to date, 7-6(5), 6-1 in the first round of Roland Garros in 2015. Stephens would go on to reach the fourth round - the last time she would reach the second week of a major before her resurgence at Flushing Meadows this year.
Madison Keys and CoCo Vandeweghe, having never played each other before this year, will now face off for the third time in just over a month. Keys won both of their US Open Series matches: 7-6(4) 6-4 in the Stanford final and 6-4, 3-6, 6-3 in the first round of Cincinnati.
Madison Keys is the only semifinalist who has won a title in 2017, having taken the trophy in Stanford. Venus Williams has been runner-up twice, at the Australian Open and Wimbledon, as well as semifinalist in Miami; CoCo Vandeweghe was the Stanford finalist, and reached one further semifinal at the Australian Open; and Sloane Stephens has notched up two semifinals, in Toronto and Cincinnati.
This is the first time in the Open Era that the US Open semifinals have not featured a Top 8 seed. The last time that no Top 4 seed reached this stage was 2004, when No.9 Svetlana Kuznetsova beat No.5 Lindsay Davenport and No.6 Elena Dementieva beat No.8 Jennifer Capriati.
2017 marks the 20-year anniversary of Venus Williams's Flushing Meadows debut, which saw the unseeded 17-year-old make it all the way to the final. That year, Williams conquered Larisa Neiland, Gala León García, No.8 seed Anke Huber, Joannette Kruger, Sandrine Testud and No.11 seed Irina Spirlea before falling to No.1 seed Martina Hingis.
The doubles quarterfinals will see a rare match pitting sisters against each other as No.2 seeds Chan Yung-Jan and Martina Hingis take on unseeded Chan Hao-Ching and Zhang Shuai. The Chans have faced each other once before, in the 2014 Kuala Lumpur final: Hao-Ching, partnering Timea Babos, took that match 6-3, 6-4 over Yung-Jan, who was paired with Zheng Saisai.
Venus Williams is 16-6 in career major semifinals, but just 4-4 at the US Open. Her victories at that stage here were over Irina Spirlea in 1997, Martina Hingis in 2000, Jennifer Capriati in 2001 and Amélie Mauresmo in 2002; and her defeats were at the hands of Lindsay Davenport in 1998, Hingis in 1999, Justine Henin in 2007 and Kim Clijsters in 2010.
Madison Keys, CoCo Vandeweghe and Sloane Stephens have each reached, and lost, one Slam semifinal - all at the Australian Open. Stephens fell to Victoria Azarenka in 2013, Keys was defeated by Serena Williams two years later, and Vandeweghe lost to Venus Williams this January.
Madison Keys has spent the least time on court of the semifinalists at seven hours and 55 minutes; CoCo Vandeweghe has spent the most, at nine hours and 34 minutes. Both Keys and Vandeweghe have dropped two sets en route to the final four, while Venus Williams and Sloane Stephens have ceded three each. Stephens has faced the highest-ranked opponents overall, with an average ranking of 30.
Venus Williams, Madison Keys and CoCo Vandeweghe are all among the Top 5 fastest servers at this year's US Open. Williams's 121mph delivery is at the top of the leaderboard; Keys is joint third with 119mph and Vandeweghe joint fifth with 118mph.
Chan Yung-Jan and Martina Hingis are the No.1 team in the Porsche Race to Singapore, having won a tour-leading six titles and compiled a 41-8 win-loss record since joining forces in February. However, a Slam title is the one accolade missing from their resumé this year: the Taiwanese-Swiss duo lost in the Roland Garros semifinals to Bethanie Mattek-Sands and Lucie Safarova, and the Wimbledon quarterfinals to Anna-Lena Groenefeld and Kveta Peschke
CoCo Vandeweghe has attributed her mental improvement to her Slam breakthroughs this year, reports Steve Tignor for Tennis.com.
The first all-American semifinal lineup at the US Open since 1981 was extremely timely, argues Peter Bodo for ESPN.
Three of this year's final four are first-time semifinalists in New York - get to know more about Madison Keys, Sloane Stephens and CoCo Vandeweghe with our WTA Insider feature.
How the #TeamAmerica final four came about for the first time since 1981.