No.3 seed and defending champion Sloan Stephens has a 2-1 advantage in her career head-to-head against Anastasija Sevastova. Included in the American's victories is a quarterfinal success at Flushing Meadows last year, where she also dispatched the Latvian at the quarterfinal stage in a thrilling contest, 6-3, 3-6, 7-6(4).
All three of the previous matches have been contested on hard courts and the pair met as recently as Montreal on August 10, when Stephens powered through, 6-2, 6-2 at the quarterfinal stage.
The No.17's seed win over the American came in Zhuhai in the latter part of 2017.
Maria Sharapova was the last No.3 seed to reach the semifinals of the US Open. The Russian achieved that in 2012. Stephens is the first to reach as far as the quarterfinals since.
Serena Williams has qualified for at least the semifinals in the last eight US Opens in which she has played, winning four of those. The last time she was beaten in the final eight was 2007 when Justine Henin defeated her 7-6(3), 6-1.
Karolina Pliskova is a former finalist in New York, having been beaten by Angelique Kerber in the 2016 final. However, she fell at the quarterfinal hurdle last year, defeated in two sets by CoCo Vandeweghe.
Remarkably, Serena and Karolina have faced each other only twice previously on Tour. They duelled in the 2016 US Open, with the Czech coming through that day, 6-2, 7-6(5). Williams had won their first meeting, 7-2, 6-2, on the hard courts of Stanford.
Beginning warm and clear but with increased cloud coverage in the middle of the afternoon and early evening. Maximum temperature: 33C/91F.
Steve Tignor of tennis.com reflects on Maria Sharapova having her perfect record under the New York lights ended by Carla Suárez Navarro.
In the New York Times, Ben Rothenberg looks at why Karolina Pliskova wants to play Serena Williams so badly.
It's time to pay attention to Madison Keys, Sandra Harwitt of USA Today argues.
ORDER OF PLAY
Check out all of Day 9's scheduled women's matches on the WTA website here.