Garbiñe Muguruza trails Petra Kvitova in their previous meetings, losing in both Beijing and Stuttgart to the Czech last year, having won their first meeting, at the WTA Finals in 2015.
Venus Williams seeks a third successive victory over Carla Suárez Navarro, having won in Wuhan in 2015 and Wimbledon last year. The pair alternated victories in their five encounters before that - Suárez Navarro winning at the 2009 Australian Open, in Rome in 2014 and Miami a year later, while Venus prevailed at Wimbledon in 2009 and in Montreal in 2014.
Maria Sharapova's next US Open match victory will push her up to outright third in the list of most US Open wins among active players. Currently on 35 before Sunday's matches, she is level with Caroline Wozniacki and Svetlana Kuznetsova. The Williams sisters, Serena (89) and Venus (74) lead the way.
Julia Goerges has just one victory over Sloane Stephens to her name, from their five previous encounters. The German's triumph on the Strasbourg clay in 2014 is sandwiched by losses to the American in Carlsbad (2011) and New Haven (2013) and also Auckland (2016) and Cincinnati (2017).
Four former Grand Slam winners are in action on Sunday. Our Insider profiles the players and looks at where today's matches may be won or lost.
Jon Wertheim of Sports Illustrated looks back at the first week of the tournament and grades the winners and losers.
Home favorite CoCo Vandeweghe enjoyed a spirited victory over Agnieszka Radwanska yesterday, as Steve Tignor of Tennis.com reports.
CoCo Vandeweghe discusses her Saturday victory and her hopes heading into the second week as she seeks a third Slam quarterfinal appearance of the year.