The head-to-head between Karolina Pliskova and CoCo Vandeweghe is all square at 2-2; each match has been won in straight sets. Pliskova won their first meeting in 2013, in Miami qualifying 6-2, 6-3, and their most recent, in the second round of Stüttgart this year 7-6(2), 6-4. Vandeweghe's victories came at Wimbledon in 2015 and Dubai in 2016.
Kaia Kanepi won her only encounter with Madison Keys 6-4, 6-3 in the first round of Madrid in 2015. The US Open is just the Estonian's fifth WTA-level main draw since that tournament.
Last night, Venus Williams and Sloane Stephens set up an all-American semifinal in the bottom half. Madison Keys and CoCo Vandeweghe have the opportunity to complete an all-American final four - the last time this happened at a Slam was at Wimbledon in 1985 when Martina Navratilova beat Zina Garrison and Chris Evert beat Kathy Rinaldi, with Navratilova winning the final. The last time this happened at the US Open was 1981, when Navratilova beat Evert and Tracy Austin beat Barbara Potter, with Austin winning the final.
At No.418, Kaia Kanepi is the lowest-ranked player to reach a major quarterfinal since 1983 (aside from unranked players), and would be the lowest-ranked player to reach the US Open semifinals since rankings were introduced. She has matched the best result by a qualifier at the US Open (Barbara Gerken in 1981) and would become just the third qualifier in history to reach a Slam semifinal (following Christine Dorey at the 1978 Australian Open and Alexandra Stevenson at Wimbledon in 1999).
CoCo Vandeweghe has saved her best tennis for the majors in 2017. Along with an Australian Open semifinal and Wimbledon quarterfinal, her Flushing Meadows run takes her Slam record to 13-3 this year so far. Her Tour record outside the Slams, however, is just 7-11, including seven opening-round defeats.
The US Open is just Kaia Kanepi's fifth tournament of 2017 and her first Tour-level main draw, but she has compiled an 18-2 win-loss record already. The Estonian is undefeated on the ITF circuit, winning events in Essen, Germany and Parnu, Estonia; her only losses have come in Wimbledon qualifying to Arina Rodionova and Bucharest qualifying to Arantxa Rus.
Four of CoCo Vandeweghe's 12 wins over Top 10 players have come this year, including her first victory over a reigning No.1 (Angelique Kerber at the Australian Open). If she beats Karolina Pliskova today for a second defeat of a No.1, it would seal the largest number of Top 10 wins the American has ever scored in a single season.
No.1 seed Karolina Pliskova is the only Top 8 seed to have reached the quarterfinals. Since 32 seeds were introduced in 2001, only twice before has the last eight featured one Top 8 seed - both times at the US Open, in 2009 (No.2 Serena Williams) and 2014 (No.1 seed Serena Williams).
Today marks the third Slam quarterfinal for Madison Keys, who is 1-1 in her previous pair; the fourth Slam quarterfinal for Karolina Pliskova and CoCo Vandeweghe, who are 2-1 and 1-2 respectively at this stage; and the sixth appearance in a major last eight for Kaia Kanepi, who has lost all five of her previous quarterfinals. This means Kanepi is the only player remaining who can extend the streak of Slams featuring a debut semifinalist to 18, begun at Wimbledon 2013 by Kirsten Flipkens.
Kanepi's 0-5 quarterfinal record ties her with Carla Suárez Navarro among active players yet to reach a semifinal, and just ahead of Anastasia Pavlyuchenkova, who is 0-4 at this stage. Kanepi's losses have come to Svetlana Kuznetsova (Roland Garros 2008), Petra Kvitova (Wimbledon 2010), Vera Zvonareva (US Open 2010), Maria Sharapova (Roland Garros 2012) and Sabine Lisicki (Wimbledon 2013); the only set she won across those matches was against Kvitova.
If Madison Keys loses to Kanepi today, it would mark her second Slam loss of 2017 to a former Top 50 player ranked beneath 200 on the comeback trail following her Roland Garros defeat by Petra Martic.
Two thrilling quarterfinals, each going to a final set tiebreak, made last night a US Open peak. Kevin Mitchell reports on two brilliant showcases of tennis for The Guardian.
The late-night classic between Venus Williams and Petra Kvitova was, WTA Insider Courtney Nguyen argues, "a gift to the sport".
Relive the highlights as Venus Williams made her way into her third Slam semifinal of 2017.