Serena Williams has reached the second week of a major for the 58th time in her career. The 23-time Slam champion is 45-9 in this round, with her last loss in the US Open round of 16 coming in 2006 at the hands of Amélie Mauresmo. Williams last faced Kaia Kanepi at the same stage of the 2014 US Open, winning 6-3, 6-3; overall, she is 4-0 against the Estonian, all straight-sets victories.
Kaia Kanepi's record in fourth-round major matches is 6-1, with the 2014 loss to Serena the only blot on her copybook at this stage. This week marks the first time that last year's quarterfinalist has reached the second week at a Slam in back-to-back years.
Defending champion Sloane Stephens faces Elise Mertens for the second time this month today. In their first meeting, in the third round of Cincinnati two weeks ago, the Belgian triumphed 7-6(8), 6-2. Mertens has now reached the second week of a major for the third time in her career, all this year, while Stephens has progressed to this stage for the 11th time in total, having compiled a 4-6 record in Slam fourth rounds.
No.8 seed Karolina Pliskova and No.19 seed Anastasija Sevastova are both bidding for a hat-trick of quarterfinals in Flushing Meadows today. The Latvian's only other second-week showing at a major was her fourth-round run at the 2011 Australian Open.
For the first time ever, two Ukrainians have progressed to the fourth round of a Grand Slam: No.7 seed Elina Svitolina and Lesia Tsurenko. Both will bid to become the first player from their country to reach a US Open quarterfinal since Kateryna Bondarenko defeated Gisela Dulko 6-0, 6-0 in 2009.
Elina Svitolina and Anastasija Sevastova have faced each other just once previously - in the first round of the Cali 125K in 2013 on clay, which the Ukrainian won 6-3, 7-6(7).
Naomi Osaka is the first Japanese woman to reach the second week in Flushing Meadows since Shinobu Asagoe's quarterfinal showing in 2004. The 20-year-old has dropped just seven games in three matches so far, and her double bagel victory over Aliaksandra Sasnovich in the third round was her first such triumph since 2013, when as a 15-year-old she defeated 66-year-old Gail Falkenberg in the first qualifying round of the Rock Hill ITF $25,000 event.
Ashleigh Barty is one of three players making their debut in the second week of a Slam, along with Aryna Sabalenka and Marketa Vondrousova. The Australian leads the head-to-head against 2016 finalist Karolina Pliskova 2-1. Tiebreaks have been a common feature of the pair's encounters: Barty won both of her victories in deciding tiebreaks, including a 4-6, 7-6(3), 7-6(2) scoreline in their most recent encounter in last year's Wuhan quarterfinals. Pliskova's win, in the 2016 Nottingham quarterfinals, was via a pair of tiebreaks.
At 19 years and two months old, Marketa Vondrousova is the youngest player to reach the fourth round at the US Open since Ana Konjuh's quarterfinal run here in 2016 at 18 years and eight months. Vondrousova also takes over from Konjuh as the youngest active player to have made the second week of a major.
A year of four different Slam champions is guaranteed for the 14th time in the Open Era following the losses of Caroline Wozniacki, Simona Halep and Angelique Kerber - the first time since 2011 that the year's three previous major winners had not made it to the second week in New York. Only once in history have there been eight different Slam champions across two years, in 1937-38: Nancye Wynne, Hilde Krahwinkel Sperling, Dorothy Round, Anita Lizana, Dorothy Cheney, Simonne Mathieu, Helen Wills Moody and Alice Marble. Unless Serena Williams or Sloane Stephens win the US Open, that record will be matched this year.